2023 seminars |
🔗 Next up |
The combination of entanglement, measurement, and classical communication allows the teleportation of quantum states between distant parties. In this talk, I will study the transfer of correlations and entanglement in many-body wavefunctions under imperfect teleportation protocols, specifically focusing on the ground state of a critical Ising chain. I will show that imperfections act as weak measurements on the teleported critical state. Leveraging the theory of measurement-altered quantum criticality, I can quantify the resilience of critical-state teleportation. Our findings identify classes of teleportation protocols that either maintain universal long-range entanglement and correlations, weakly modify them, or preserve power-law correlations while eliminating long-range entanglement. | ||
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Tuesday, December 12, 2023 12:00 (Rome) room 128 (SISSA, via Bonomea) |
Sara Murciano |
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Quantum criticality under imperfect teleportation |
Randomized measurements provide a practical scheme to probe complex many-body quantum systems. While they are very powerful to extract local information, global properties such as bipartite entanglement remain hard to probe, requiring a number of measurements or classical post-processing resources growing exponentially in the system size. In this talk, I will address the problem of estimating pure and mixed-state entanglement via partial-transposed (PT) moments, and show that efficient estimation strategies exist under assumptions which are very natural from the point of view of quantum simulation, namely that all the spatial correlation lengths are finite. Focusing on 1D systems, I will introduce a set of approximate factorization conditions on the system density matrix and show that the latter allow us to reconstruct entanglement entropies and PT moments from information on local subsystems. Combined with the RM toolbox, this yields a simple strategy for entanglement estimation which is provably accurate and which only requires polynomially-many measurements and post-processing operations. I will discuss the generality of the approximate factorization conditions, showing their validity in large classes of many-body states, including matrix-product density operators and thermal states of local Hamiltonians. I will argue that the proposed method could be practically useful to detect bipartite mixed-state entanglement for large numbers of qubits available in today’s quantum platforms. | ||
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Tuesday, December 12, 2023 11:00 (Rome) room 128 (SISSA, via Bonomea) |
Lorenzo Piroli |
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Many-body entanglement from polynomially-many measurements |
I will discuss how a non-invertible (topological) symmetry explains why the flow from the dilute to the dense phase in loop models is robust and generic, hence solving a small old puzzle. | ||
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Tuesday, December 5, 2023 11:00 (Rome) room 128 (SISSA, via Bonomea) |
Hubert Saleur |
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Non-invertible symmetries in loop soups and applications |
I will present recent works on measurement protocols for two probes of many-body quantum chaos; (i) The spectral form factor (SFF) and (ii) Out of time ordered correlators (OTOC). SFF characterizes statistics of energy eigenvalues, making it a key diagnostic of many-body quantum chaos. In addition, partial spectral form factors (PSFFs) can be defined which refer to subsystems of the many-body system. They provide unique insights into energy eigenstate statistics of many-body systems. We propose a protocol that allows the measurement of the SFF and pSFFs in quantum many-body spin models, within the framework of randomized measurements. For measurements of OTOCs, we utilize properties of thermo field double states. In both the cases, experimental results will be presented. | ||
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Tuesday, November 28, 2023 11:00 AM (Rome) room 138 (SISSA, via Bonomea) |
Lata Kharkwal Joshi |
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Measurements of many-body quantum chaos |
Out-of-equilibrium states of many-body systems tend to evade a description by standard statistical mechanics, and their uniqueness is epitomized by the possibility of certain long-range correlations that cannot occur in equilibrium. In quantum many-body systems, coherent correlations of this sort may lead to the emergence of remarkable entanglement structures. In this talk, I will present exact analytical results concerning entanglement within the steady state of free fermions that occupy a one-dimensional lattice containing a noninteracting impurity, and that are subjected to an external bias by two edge reservoirs. I will show that two subsystems located on opposite sides of the impurity, and within a similar distance from it, exhibit volume-law entanglement regardless of their separation, as measured by their fermionic negativity. The mutual information of the subsystems, which quantifies the total (classical and quantum) correlations between them, follows a similar scaling. This behavior arises whenever the energy distribution functions of the two edge reservoirs differ, thus capturing both the case of a chemical-potential bias and the case of a temperature bias (as well as any combination of the two). The extensive terms of the negativity and mutual information feature a simple and universal functional dependence on the scattering probabilities associated with the impurity, a functional dependence which has a clear interpretation in terms of the coherence generated between the transmitted and reflected parts of scattered wavepackets. To the extent that time permits, I will discuss further exact results for the subleading corrections to these asymptotic expressions in the case of zero temperature.
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Tuesday, November 21, 2023 11:00 (Rome) room 128 (SISSA, via Bonomea) |
Shachar Fraenkel |
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Extensive long-range entanglement in a nonequilibrium steady state |
We will consider spin chains with “medium-range” interactions, i.e. beyond the interaction of neighbouring nodes. At first glance, such systems are a natural and trivial generalization of the Heisenberg magnet, thus they have been little explored in the past. However, they are of significant interest due to their connection to cellular automata on the one hand and the obtaining of new solutions to the Yang-Baxter equation on the other. We will separately discuss the connection of such a generalization with the TT-bar deformation in QFT. | ||
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November 14, 2023 11.00 room 138, SISSA + Virtual |
Artur Hutsalyuk |
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Spin chains with “medium-range” interactions and cellular automata |
Whether chiral topological quantum states in (2+1)d can be represented by Projected Entangled Pair States (PEPS) is a fundamental question. A noted result (due to Wahl, Tu, Schuch, and Cirac, and Dubail and Read) says that this is possible for non-interacting fermions, but the answer is as yet unknown for interacting systems. Characteristic counting of degeneracies of low-lying states in the entanglement spectrum (ES) at fixed transverse momentum of bipartitioned long cylinders (“Li-Haldane counting”) provides often-used supporting evidence for chirality. In this talk, I will discuss our work on understanding the ES beyond Li-Haldane counting in the case of ground states of SU(2) and SU(3) chiral spin liquids. We examine the splittings of degeneracies in entanglement energy, which are determined by the generalized Gibbs ensemble (GGE) of local conserved quantities from CFT that respect the symmetries of the system. This information serves as a finer diagnostic (than the countings alone) of chiral topological behavior. It turns out that non-chiral states (with zero chiral central charge), yet strongly breaking time-reversal and reflection symmetries (i.e., “apparently” chiral states), are known, whose low-lying ES exhibits the same Li-Haldane counting as a chiral state (with non-zero chiral charge) in certain topological sectors. By studying the splittings of the ES in an Abelian SU(3) spin liquid PEPS, we recently identified a hallmark of chirality of a wave function: The exact degeneracies of entanglement-energy levels in the ES corresponding to paired conjugate representations, which are split in non-chiral states. | ||
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November 7, 2023 11.00 room 138, SISSA + Virtual |
Mark Arildsen |
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Beyond Li-Haldane Counting: A Close Look at Chiral Topological Order in the Entanglement Spectra of (2+1)-Dimensional Spin Liquid Ground States, with a Focus on PEPS |
Super-additivity, concavity, and homogeneity of the entropy function set up the basis for the maximum entropy principle, which arises from the second law of thermodynamics. On the other hand, in the case of non-additive systems, which are common in the presence of long-range interactions and in black hole thermodynamics, the entropy is not a homogeneous but a quasi-homogeneous function. Although some fundamental results, such as the zeroth law of thermodynamics and the Gibbs-Duhamel relationship, have already been established for quasi-homogeneous entropies, the relationships between quasi-homogeneity, concavity, and super-additivity for non-additive systems are still unknown. In this talk, we will discuss these relationships and the results will be applied to the characterization of the generalized Landsberg ideal gas, which is an example of a simple system that violates homogeneity (and consequently super-additivity) while being consistent with ideal gas state equations. Possible applications for the characterization of long-range interacting systems will also be discussed. | ||
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September 12, 2023 11.00 room 138, SISSA + Virtual |
Velimir Ilić |
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Super-additivity, generalized concavity and quasi-homogeneity in non-additive systems |
Driven-dissipative quantum fluids of light, experimentally realised in for example semiconductor microcavities, circuit or cavity QED systems, provide a unique testbed to explore new non-equilibrium quantum phenomena. I will review recent progress in this field. In particular, we show that polariton quantum fluid can exhibit a non-equilibrium order, where superfluidity is accompanied by stretched exponential decay of correlations [1]. This celebrated Kardar-Parisi-Zhang (KPZ) phase has not been achieved before in any system in 2D, and even 1D realisations have not been conclusive. I will then discuss how these systems can undergo other unconventional phase transitions and orders [2], and display flow properties connected but distinct from conventional superfluidity. Finally, when placed in strained honeycomb lattice potentials, polariton fluids can condense into a rotating state, the lowest Landau level, forming a vortex array and spontaneously breaking time reversal symmetry [3]. Describing strong quantum correlations in open systems with drive and dissipation, especially in two dimensions, is a numerical challenge. I will briefly present our attempts at developing suitable methods, based on stochastic and tensor network approaches [4].
[1] A. Zamora et al, PRX 7, 041006 (2017); PRL 125, 265701 (2020); A. Ferrier et al, PRB 105, 205301 (2022) [2] G. Dagvadorj et al, PRL 130, 136001 (2023); PRB 104, 165301 (2021) [3] C. Lledo et al, SciPost 12, 068 (2022) [4] C. Mc Keever et al, PRX 11, 021035 (2021); P. Deuar et al, PRX Quantum, 2, 010319 (2021) |
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July 11, 2023 11.30 room 128, SISSA + Virtual |
Marzena H. Szymanska |
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Novel Non-equilibrium Phenomena in Quantum Fluids of Light |
Integrable quantum field theories in 1+1 dimensions provide beautiful examples of exactly solvable models. Relying on the existence of higher-spin conserved charges characterising these models, the axiomatic S-matrix bootstrap program was applied in the past decades to conjecture analytical expressions for the S-matrices of several integrable theories in 1+1 dimensions. Integrable theories can often also be studied perturbatively using standard Feynman diagrams, where the integrability manifests itself in a priori surprising cancellations and simplifications whose underlying mechanism is not completely understood. In this talk, I will discuss these simplifications at the tree and loop level for a famous class of integrable models, the simply-laced affine Toda field theories, and I will show how these simplifications play an important role to explain the pole structure of the S-matrices of these models |
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June 20, 2023 14.30 room 128, SISSA + Virtual |
Davide Polvara |
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Higher-order singularities in S-matrices from tree-level integrability |
In May 2021, ETH Zurich and the Paul Scherrer Institute (PSI) established the ETH-PSI Quantum Computing Hub to advance quantum technologies. The hub brings together trapped ions and superconducting qubits to scale current quantum platforms to larger quantum systems of tens of qubits, exploring the synergy of different technologies. In the first part of this talk, I will present the first room-temperature setup we built designed for the individual control of up to 50 trapped-ion qubits with high fidelity. This system houses a monolithic, segmented 3D ion trap fabricated through laser-enhanced etching of fused silica and makes use of laser-written waveguides to individually control each qubit. In the second part, I will discuss preliminary results obtained at JILA working with Rydberg atoms in optical tweezer arrays. The low-vibration cryostat and a high optical access vacuum chamber enable us to develop a novel system that leverages scalable, ultracold Rydberg atom arrays for programmable quantum computation. The high-optical access vacuum chamber will allow us to create and control a large array using 3D optical lattices with site-resolved addressability and interaction control aided by optical tweezers. We will harness a bi-chromatic magic lattice to provide identical confinement for both ground and Rydberg states. |
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June 13, 2023 11.00 (Rome) Luigi Stasi Seminar Room, ICTP + virtual |
Matteo Marinelli |
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Towards High-Fidelity Control of Individual Qubits in Large Atomic Arrays |
For many quantum mechanical applications dissipation is often regarded as an undesirable yet unavoidable consequence because it potentially degrades quantum coherences and renders the system classical. However, interactions with the environment can also be considered a fundamental resource for striking collective effects typically impossible in Hamiltonian systems. A hallmark of such collective behavior in nonequilibrium systems is the phenomenon of synchronization: in the complete absence of any time-dependent forcing from the outside, a group of oscillators adjusts their frequencies such that they spontaneously oscillate in unison. With the recent developments in quantum technology which allow one to exquisitely tailor both the system and environmental properties, synchronization has emerged in the quantum domain with various different examples ranging from nonlinear oscillators to spin-1 systems, superconducting qubits and optomechanics. However, to observe synchronization in large networks of classical or quantum systems demands both excellent control of the interactions between nodes and accurate preparation of the initial conditions due to the involved nonlinearities and dissipation. This limits its applicability for future devices. In this talk, I will present a potential route towards significantly enhancing the robustness of synchronized behavior in open nonlinear systems that utilizes the power of topological insulators, which exhibit an insulating bulk but conducting surface states, known as topological edge states. These edge states display a surprising immunity to a wide range of local deformations and even circumvent localization in the presence of disorder. By combining nontrivial topological lattices with nonlinear oscillators, we show that synchronized motion emerges at the lattice boundaries in the classical (mean field) as well as the quantum regime. Furthermore, the synchronized edge modes inherit the topological protection known from closed systems with remarkably robust dynamics against local disorder and even random initial conditions. Our work demonstrates a general advantage of topological lattices in the design of potential experiments and devices as fabrication errors and longterm degradation are circumvented in this way. This is especially important in networks where specific nodes need special protection. | ||
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Tuesday, June 6 , 2023 11.00 (Rome) (Luigi Stasi Seminar Room + zoom) |
Christopher W. Wächtler |
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Topological synchronization of classical and quantum systems |
Soon after the discovery of high temperature superconductivity in the cuprates, Anderson proposed a connection to quantum spin liquids. But observations since then have shown that the low temperature phase diagram is dominated by conventional states, with a competition between superconductivity and charge-ordered states which break translational symmetry. We employ the “pseudogap metal” phase, found at intermediate temperatures and low hole doping, as the parent to the phases found at lower temperatures. We argue that the pseudogap is associated with a spin liquid, and that a particular spin liquid (the “pi-flux” state with an emergent SU(2) gauge field) exhibits confining instabilities which can resolve a number of open puzzles on the cuprate phase diagram.
This talk is based on Maine Christos, Zhu-Xi Luo, Henry Shackleton, Ya-Hui Zhang, Mathias S. Scheurer, and S. S., arXiv:2302.07885 |
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Monday, May 29, 2023 11:30 am (CET) Luigi Stasi Seminar Room ( ICTP, Leonardo Building, second floor) |
Subir Sachdev |
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Spin liquids and the phase diagram of the cuprates |
Phase transitions occur in a variety of settings and situations in particle physics, condensed matter and statistical mechanics. The traditional approach to phase transitions, based on elegant principles such as scaling and universality, assumes some basic a priori knowledge of the physical system, in particular of the Hamiltonian and its underlying symmetries. While this information is usually given for granted, a data-driven approach has wider applicability. In this talk, I will present some applications of machine learning starting from Monte-Carlo generated data for phase transitions in simple (but far from trivial!) statistical and quantum field theoretical systems. More in details, I will discuss (a) identification of symmetries; (b) constructions of order parameters; and (c) precise determination of critical temperature and critical exponents. I will conclude showing how machine learning can be used to invert a renormalisation group flow. | ||
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Tuesday, May 16, 2023 14:00 Luigi Stasi Seminar Room (ICTP, Leonardo Building, second floor) |
Biagio Lucini |
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Investigating phase transitions with machine learning methods |
In quantum many body systems symmetries lead to conservation laws, and if a specific system has exotic symmetries, then its dynamics will not be ergodic. We discuss a special phenomenon, where a quantum spin chain has exotic symmetries described by Matrix Product Operators. Such symmetries have been known to exist in integrable spin chains, where they are given by the commuting set of transfer matrices. However, in our case we find a non-commutative symmetry algebra in a non-integrable spin chain. This leads to exponentially large degeneracies, the breakdown of ergodicity and thermalization. The mechanism behind the symmetries seems to be new: it is related to the ,,hard rod deformation” discovered recently, which is a general framework that was first seen specifically in the so-called folded XXZ model, which is integrable. However, the new models include non-integrable perturbations as well, meanwhile conserving the large symmetry algebra. | ||
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Tuesday, May 9, 2023 11:00 (Rome) room 128 (SISSA, via Bonomea) |
Balázs Pozsgay |
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Ergodicity breaking and Matrix Product Operator Symmetries for a non-integrable spin chain |
Recently the interest to non-Hermitian disordered models has been revived, due to the claims of instability of a many-body localization to a coupling to a bath. To describe such open quantum systems, one often focuses on an energy leakage to a bath, using effective non-Hermitian Hamiltonians. A well-known Hatano-Nelson model [1], being a 1d Anderson localization (AL) model, with different hopping amplitudes to the right/left, shows AL breakdown, as non-Hermiticity suppresses the interference. Unlike this, we consider models with the complex gain-loss disorder and show that in general these systems tend to localization due to non-Hermiticity. First, we focus on a non-Hermitian version [2] of a Rosenzweig-Porter model [3], known to carry a fractal phase [4] along with the AL and ergodic ones. We show that ergodic and localized phases are stable against the non-Hermitian matrix entries, while the fractal phase, intact to non-Hermiticity of off-diagonal terms, gives a way to AL in a gain-loss disorder. The understanding of this counterintuitive phenomenon is given in terms of the cavity method and in addition in simple hand-waving terms from the Fermi’s golden rule, applicable, strictly speaking, to a Hermitian RP model. The main effect in this model is given by the fact that the generally complex diagonal potential forms an effectively 2d (complex) distribution, which parametrically increases the bare level spacing and suppresses the resonances. Next, we consider a power-law random banded matrix ensemble (PLRBM) [5], known to show AL transition (ALT) at the power of the power-law hopping decay a=d equal to the dimension d. In [6], we show that a non-Hermitian gain-loss disorder in PLRBM shifts ALT to smaller values $d/2 In order to analytically explain the above numerical results, we derive an effective non-Hermitian resonance counting and show that the delocalization transition is driven by so-called "bad resonances", which cannot be removed by the wave-function hybridization (e.g., in the renormalization group approach), while the usual "Hermitian" resonances are suppressed in the same way as in the non-Hermitian RP model. [1] N. Hatano, D. R. Nelson, "Localization Transitions in Non-Hermitian Quantum Mechanics", PRL 77, 570 (1996). |
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May 2, 2023 11.00 AM (Rome) Luigi Stasi Seminar Room (ICTP, Leonardo Building, second floor) + virtual |
Ivan Khaymovich |
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Localization enhancement in gain-loss non-Hermitian disordered models |
Many complex systems in Nature, from metabolic networks to ecosystems, appear to be poised at the edge of stability, hence displaying enormous responses to external perturbations. This marginal stability condition is often the consequence of the complex underlying interaction network, which can induce large-scale collective dynamics, and therefore critical behaviors. In this talk, I will address some of the still open questions — from the configurational landscape analysis to stability criteria up to the emergence of spatial disordered patterns — by discussing the Generalized Lotka-Volterra model in the limit of many randomly interacting species and finite demographic noise. Leveraging on techniques rooted in the spin-glass and random matrix theory, I will unveil a very rich structure in the organization of the equilibria [1] and relate the slowing down of correlation functions to glassy-like features [1-2]. Finally, I will discuss possible generalizations to non-logistic growth behavior [3-4]. On the one hand, this will lead to astonishing stabilization mechanisms to be framed within the long-standing diversity-stability debate initiated by May. On the other hand, these developments will allow us to describe positive feedback mechanisms and eventually pinpoint new phase transitions as a smoking-gun signature of criticality. |
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Thursday, April 27, 2023 14:30 (Rome) Room 128, SISSA |
Ada Altieri |
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Complexity of large ecological communities through the lens of statistical physics |
I will describe how to use integrability techniques to compute the spectrum of certain string theories, which are important in the context of the holographic principle. The talk will be based on arXiv:2303.02120. | ||
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Thursday, April 20, 2023 14:30 (Rome) Room 138, SISSA |
Alessandro Sfondrini |
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The spectrum of string theory from integrability |
A quantum integrable system slightly perturbed away from integrability is typically expected to thermalize on timescales of order τ∼λ-2, where λ is the perturbation strength. We here study classes of perturbations that violate this scaling, and exhibit much longer thermalization times τ∼λ-2k where k>1 is an integer. Systems with these “weak integrability breaking” perturbations have an extensive number of quasi-conserved quantities that commute with the perturbed Hamiltonian up to corrections of order λk. We demonstrate a systematic construction to obtain families of such weak perturbations of a generic integrable model for arbitrary k. We then apply the construction to various models, including the Heisenberg, XXZ, and XYZ chains, the Hubbard model, models of spinless free fermions, and the quantum Ising chain. Our analytical framework explains the previously observed evidence of weak integrability breaking in the Heisenberg and XXZ chains under certain perturbations. | ||
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Tuesday, April 18, 2023 11:00 (Rome) Luigi Stasi Seminar Room (Leonardo Building, ICTP) |
Federica Surace |
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Weak perturbations of integrable models |
Understanding the effect of correlations in interacting many-body systems is one of the main challenges in quantum mechanics. While the general problem can only be addressed by approximate methods and numerical simulations, in some limiting cases it is amenable to exact solutions. In this talk I will present a family of exact solutions which allows to obtain the many-body wavefunction of strongly correlated quantum fluids confined by a tight waveguide and subjected to any form of longitudinal confinement. It directly describes the experiments with trapped ultracold atoms where the strongly correlated regime in one dimension has been achieved. The exact solution applies to bosons, fermions and mixtures. In particular I will discuss (i) the role of symmetries and the possibility to obtain an exact solution even for systems in which the SU(k) symmetry is broken; (ii) the interplay between a hard wall trapping potential and correlations; and (iii) the spin mixing dynamics in a fermionic mixture confined in a harmonic potential. | ||
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Tuesday, April 4, 2023 11:00 (Rome) Room 138, SISSA |
Patrizia Vignolo |
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Exact solutions for 1D trapped strongly correlated systems: symmetries, trapping effects and dynamics |
In the talk I will present a new theory to access large scale correlations and fluctuations in generic non-stationary and inhomogeneous situations, where hydrodynamic motion takes place. This theory is inspired by the well-known macroscopic fluctuation theory (MFT). The latter is the hydrodynamic large-deviation theory, which has been successfully applied to a variety of classical systems, for fluctuations beyond mean values of purely diffusive fluxes. The “ballistic macroscopic fluctuation theory’’ (BMFT) focuses, instead, on the Euler scale, which exists and is non trivial for systems, classical or quantum, admitting ballistic transport. The BMFT requires as input solely the Euler hydrodynamic data of the system. It is a large-deviation theory formulated on the basis of an action principle over hydrodynamic space-time trajectories from which fluid correlations and current fluctuations can be derived. Strikingly, the theory predicts the novel effect that initially uncorrelated fluid regions develop over time long-range correlations, which invalidate, for correlations, the local relaxation assumption at the basis of the description of hydrodynamics’mean values. | ||
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Tuesday, March 21, 2023 11:00 (Rome) room 128 (SISSA, via Bonomea) |
Gabriele Perfetto |
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Large-scale correlations and fluctuations from the ballistic macroscopic fluctuation theory |
Crossing a continuous phase transition results in the formation of topological defects with a density predicted by the Kibble-Zurek mechanism (KZM). We report on two predictions beyond KZM:
First, it is shown that the statistics of defects follow a binomial distribution with N Bernoulli trials associated with the probability of forming a topological defect at the locations where multiple domains merge. All cumulants of the distribution are predicted to exhibit a common universal power-law scaling with the quench time in which the transition is crossed. Knowledge of the distribution is used to discuss the onset of adiabatic dynamics and bound rare events associated with large deviations. Second, we characterize the spatial distribution of point-like topological defects in the resulting nonequilibrium state and model it using a Poisson point process in arbitrary spatial dimensions with KZM density. In one-dimensional systems, defect-defect correlations are enhanced and can be taken into account by considering the finite size of defects. The theory is expected to accurately reproduce the spacing distribution in higher dimensions, as we have shown in a two-dimensional setting, where the remaining deviations are attributed to coarsening. Our results are amenable to experimental tests with established technology, exploiting any of the platforms previously used to probe KZM scaling, provided it is endowed with spatial resolution, as is the case with trapped-ion systems, colloidal monolayers, multiferroics, ultracold gases in various geometries, and quantum simulators, to name some prominent examples. |
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March 7, 2023 11.00 (Rome) room 138 (SISSA, via Bonomea) + virtual |
Adolfo Del Campo |
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Beyond the Kibble-Zurek mechanism |
The eigenstate thermalization hypothesis, as developed by Berry, Deutsch and Srednicki, concerns the average and variance of the matrix elements of physical operators in the eigenbasis of the Hamiltonian. It turns out that the assumption of statistical independence of these elements is not consistent, and ETH has to be supplemented with the higher correlations. In doing so, one encounters a relation with the theory of “Free Probability”, which I will describe. | ||
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February 28, 2023 14.30 (Rome) room 138 (SISSA, via Bonomea) + virtual |
Jorge Kurchan |
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The complete form of the Eigenstate Thermalization Hypothesis (ETH) and Free Probability |
In this talk, I will provide a gentle introduction to quantum estimation theory, highlighting key concepts and methods, such as quantum and classical Fisher information, Cramer-Rao bound, squeezing, etc. In particular I will discuss the role of entanglement to reduce the uncertainty in the estimation of a single and multiple parameters. I will show how quantum parameter estimation, beside being of technological relevance, is also a highly interdisciplinary research topic, with connection to quantum geometry, quantum information, many-body physics, etc.. | ||
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February 21, 2023 11:00 AM (Rome) room 128 (SISSA, via Bonomea) |
Luca Pezzè |
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Entanglement in multiparameter quantum estimation |
Amperean superconductivity is an exotic phenomenon stemming from attractive effective electron-electron interactions (EEEIs) mediated by a transverse gauge field. Originally introduced in the context of quantum spin liquids and high-Tc superconductors, Amperean superconductivity has been recently proposed to occur at temperatures on the order of 1-20 K in two-dimensional, parabolic-band, electron gases embedded inside deep sub-wavelength optical cavities. In this talk, I first generalize the microscopic theory of cavity-induced Amperean superconductivity to the case of graphene and then argue that this superconducting state cannot be achieved in the deep sub-wavelength regime. In the latter regime, indeed, a cavity induces only EEEIs between density fluctuations rather than the current-current interactions which are responsible for Amperean pairing. | ||
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January 31, 2023 11:30 AM (Rome) Luigi Stasi Seminar Room (ICTP) + virtual |
Gian Marcello Andolina |
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Can deep sub-wavelength cavities induce Amperean superconductivity in a 2D material? |
Recent work has revealed a host of “dualities” between strongly interacting models. As apparent from the canonical example of Kramers and Wannier, such dualities are much subtler than a one-to-one mapping of energy levels, but rather are non-invertible. I explain such dualities between four distinct 1d quantum Hamiltonians, including the XXZ chain. I describe a new algebraic structure, and exploit it to find and derive non-invertible maps to three other “dual” integrable Hamiltonians. The new models describe Rydberg-blockade bosons on the ladder constrained to at most one particle per square, a three-state antiferromagnet, and two Ising chains coupled in a zig-zag fashion. The maps are developed using topological defects coming from fusion categories and the lattice analog of the orbifold construction. | ||
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January 24, 2023 11:00 AM (Rome) Room 128 (SISSA, Via Bonomea) + virtual |
Paul Fendley |
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From the XXZ chain to the integrable Rydberg-blockade ladder via non-invertible duality defects |
Topological signals associated not only to nodes but also to links and to the higher dimensional simplices of simplicial complexes are attracting increasing interest in machine learning and network science. However, little is known about the collective dynamical phenomena involving topological signals. In this talk, I will introduce the Hodge Laplacian and the topological Dirac operator that can be used to process simultaneously topological signals of different dimensions. I will discuss the main spectral properties of the Dirac operator defined on networks and simplicial complexes. I will present the topological Dirac equation in which the spinor has a geometrical interpretation and is defined on both nodes and links of the network and the potential implications of this model. I will show that topological signals treated with the Hodge Laplacians or with the Dirac operator can undergo collective synchronization phenomena displaying different types of critical phenomena. The coherent state of these processes are localized on the higher-dimensional cavities of the simplicial complex opening new perspectives to characterize the interplay between topology and dynamics. | ||
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January 17, 2023 11:30 AM (Rome) Budinich Lecture Hall, ICTP |
Ginestra Bianconi |
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The Dirac operator and the dynamics of topological signals |
In this talk we study the out-of-equilibrium dynamics of an integrable quantum field theory possessing an unstable excitation in its spectrum. In the standard scattering picture, unstable particles result from complex poles of the two-particle scattering matrix located in the unphysical sheet of rapidity space. Because of their finite life-time, they are not part of the asymptotic particle spectrum, and their presence is only felt through the effect they have on physical quantities associated to the stable constituent particles (i.e. energy/particle densities). These quantities can be computed by employing the Generalised Hydrodynamic approach. We will see that, for an initial state given by a spacial Gaussian profile of temperatures peaked at the origin, time evolution gives rise to particle and spectral particle densities that exhibit hallmarks of the creation and decay of unstable particles. We will quantify these signatures of instability and aim to provide a more physical picture of the dynamics of unstable particles that goes beyond the pole structure of the scattering matrix. | ||
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January 10, 2023 11.00 AM (Rome) Room 128 (SISSA) + virtual |
Cecilia De Fazio |
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Generalised Hydrodynamics of Particle Creation and Decay |
2022 seminars |
We present a Wigner function-based approach for the particle density evolution in fermionic and bosonic open quantum many-body systems, including the effects of dephasing. In particular, we focus on chains of non-interacting particles coupled to Lindblad baths. The dissipative processes, described by linear and quadratic jump operators, are modulated by inhomogeneous couplings. Following a semi-classical approach, we find the differential equation governing the Wigner function evolution, which can be solved in closed form in some particular cases. We check the accuracy of the Wigner approach in different scenarios (i.e. Gaussian jump frequencies), describing the density evolution and the transport phenomena in terms of classical quasi-particles. | ||
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December 13, 2022 11.00 AM (Rome) room 138 SISSA + Virtual |
Michele Coppola |
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Wigner dynamics for Fermi gases under inhomogeneous gain and loss processes with dephasing |
Describing full unitary dynamics of a many-body system is difficult and also unpractical. Focusing on a coarse-grained dynamics or few select observables often results in a more compact non-unitary evolution. Studying bipartite entanglement dynamics in random circuits one can derive a Markovian transfer matrix description that harbors rather intriguing many-body non-Hermitian physics. The speed of generating entanglement is not given by the 2nd largest eigenvalue of the transfer matrix, but rather by a phantom eigenvalue — an eigenvalue that is not in the spectrum of any finite transfer matrix. Resolution of this paradox will lead to a pseudospectrum and a realization that, when dealing with finite non-Hermitian matrices it can be that being exact is actually wrong, while being slightly wrong is correct. | ||
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November 29, 2022 11:30 AM (Rome) Luigi Stasi Seminar Room ICTP + virtual |
Marko Znidaric |
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Two-step relaxation in many-body systems |
The past 20 years have solidified quantum entanglement as a research topic of continued and central interest for physicists working in domains as diverse as high energy physics, condensed matter theory and quantum information. Most relevantly for this talk, entanglement measures have turned out to be a powerful probe into the physics of 1D quantum critical systems.
Most known results for such setups address the entanglement in critical quantum systems with periodic BC. For open systems, there are results for the Rényi entropies and other entanglement measures of an interval touching one of the boundaries with the rest of the system. However, for more generic bipartitions or mixed boundaries, few exact calculations have been completed. In this talk, I will show how the second Rényi entropy of an interval disconnected from the boundary can be computed exactly, through CFT methods, provided the same conformal boundary condition is applied on both sides. This will be followed by a comparison with spin chain numerics and a discussion of finite-size effects. Finally, I will also succinctly present some results for Rényi entropies in systems with mixed boundaries. |
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November 25, 2022 11:00 AM (Rome) Room 138 SISSA + virtual |
Andrei Rotaru |
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Rényi entropies in one-dimensional quantum critical systems with open boundaries |
The large-anisotropy limit of the anisotropic Heisenberg spin-1/2 chain gives rise to the so-called folded XXZ model. This talk will mainly be a summary of what is and what is not yet known about this kinetically constrained model, and of a rich palette of phenomena that emerge in it. I will provide a brief explanation of the folded model’s origin, of its symmetries, of the quasiparticle content constituting its Bethe Ansatz solution, and of what is still not understood. I will explain how the symmetries emergent in the large anisotropy limit of the Heisenberg model result in a rich Hilbert space structure, which exhibits weak fragmentation and a Fibonacci sector of jammed configurations. Jammed states in the latter sector exhibit a somewhat counterintuitive dynamical phenomenon, in which a localised perturbation can have everlasting effects. Finally, if time permits, I will also briefly comment on the subject of semilocal charges that arise in the folded XXZ model and similar kinetically constrained models. | ||
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November 22, 2022 11:00 AM Rome Room 138 SISSA + virtual |
Lenart Zadnik |
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The folded XXZ model |
In recent years, the promise of a large-scale programmable quantum computer became more and more concrete at an impressive pace, but the final goal is still beyond the horizon. In the meanwhile, analogic quantum simulators provide highly tunable tabletop realizations of exciting phenomena traditionally belonging to high-energy physics, or admittedly simplified versions of them. In this talk, I will discuss how confinement of excitations can be realized in spin chains and analyze some facets of their rich nonequilibrium dynamics, ranging from short-time features to atypically slow late-time thermalization. | ||
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November 15, 2022 11.00 AM Rome Room 138 SISSA + virtual |
Alvise Bastianello |
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Many-body dynamics in confined spin chains |
In the last few years, much attention has been devoted to the study of a peculiar class of irrelevant deformations of 2-dimensional Quantum Field Theories, known as “Solvable Irrelevant Deformations”. The poster child of these is the celebrated “TTbar deformation”. They display unusual properties in the UV, which can be described exactly, their irrelevant nature notwithstanding. For this reason they represent a sensible extension of Quantum Field Theory beyond the Wilsonian paradigm and have attracted a considerable attention from the high energy theory community. The property of being solvable is shared with a wider class of deformations, constructed out of pairs of conserved currents. In general these are marginal deformations, thus presenting very different UV properties. Nonetheless their structures are similar to the TTbar ones, hinting at the existence of a universal description. In this talk I will present a very general framework that accommodates both solvable irrelevant and solvable marginal deformations, which amounts to a “topological gauging” of the symmetries of the system. Through simple path integral computations, I will recover the main features of these theories and show their equivalence to TST and Yang-Baxter deformations. For the case of TTbar, the topological gauging perspective explains the previously not understood relation to field theory in non-commutative Minkowski space-time and to the centrally extended Poincaré algebra. |
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November 8, 2022 11.00 AM Rome Room 005 SISSA + virtual |
Stefano Negro |
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Topological gauging and non-relevant deformations of Quantum Field Theories |
In quadratic Hamiltonians, various quantum correlation measures such as entanglement entropy, fidelity, and Loschmidt echo possess an inherent scaling symmetry that the Hamiltonian of the system does not have. We exploit this symmetry to address various problems in quantum field theory and semi-classical gravity. To begin with, it helps us attribute any occurrence of entropy divergence, even in the UV limit, to the generation of zero-modes in time-independent systems. For (1+1)-dimensional massive scalar fields, the scaling symmetry also provides a way to understand the crossover near the zero-mode limit. The scaling symmetry can be further generalized to time-dependent systems. Such systems may evolve to develop inverted or zero-mode instabilities that potentially apply to various physical phenomena. We show that, asymptotically, in the presence of instabilities, the leading order dynamics of various correlation measures — entanglement entropy, fidelity, and Loschmidt echo — are related via simple expressions. We quantify such instabilities in terms of scrambling time and Lyapunov exponents and show that the system mimics classicality under certain conditions in the chaotic regime. We also show that the entropy scaling oscillates between the area-law and volume-law for a scalar field that undergoes a global quench. We then discuss its implications for the quantum-classical transition of primordial density perturbations in the early Universe.
[Refs: https://arxiv.org/abs/2205.13338, https://doi.org/10.1103/PhysRevD.103.125008, https://doi.org/10.1103/PhysRevD.102.125025 ] |
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September 27, 2022 11.00 AM Rome Room 138 (SISSA) + Virtual |
Mahesh Chandran |
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Scaling symmetry of quantum correlations in quadratic Hamiltonians and its applications |
There is an old temptation to formally extend notions of entropy to nonequilibrium systems (e.g. as (neg)information as is somewhat fashionable today). In the original formulation, equilibrium entropy refers to heat transfer (Clausius) and to (energetic) spectra (Boltzmann-Planck), and has been foundational to equilibrium statistical mechanics for giving the probability of fluctuations and the structure of linear response. To reconciliate in one entropy these various tasks for nonequilibrium systems as well sadly fails, except when close-to-equilibrium. Yet, tigers are far from equilibrium. The good news is that nonequilibrium heat capacities may still inform us about thermal properties of driven and active systems. They pick up kinetic (frenetic) contributions, going much beyond thermodynamic information. | ||
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September 14, 2022 11:00 AM Rome Room 005 (SISSA) + virtual |
Christian Maes |
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What is the entropy of a tiger? |
In quantum mechanics, the role of an observer is fundamentally different from that of a classical observer. The quantum mechanical observer necessarily plays an active role in the dynamics of the system that it is observing. This apparent difficulty may be turned into a tool to drive an initially trivial system into a complicated quantum many-body state simply by observing it. I will present two remarkable examples of states induced by measurement. In the first, we examine the role of a moving density measuring device interacting with a system of fermions, and in particular, show that it would leave behind a wake of purely quantum origin. In the second example, inspired by topological Floquet insulators, we will see how a suitably chosen set of density measurements, repeated periodically, will induce robust chiral edge motion of fermions. These examples show how quantum mechanical observation can be added as a versatile tool to the arsenal of quantum engineering in condensed matter systems. | ||
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July 26, 2022 11:00 AM Rome Room 138 (SISSA) + virtual |
Israel Klich |
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Stirring by staring: non-equilibrium states by measurements in quantum systems |
The Sinh-Gordon model is a 1+1 dimensional quantum field theory with a potential cosh(b phi) that is quite peculiar. It is at the same time exactly solvable (for many observables) and not well understood. I will present the results of a variational exploration of its strong coupling regime with a recent generalization of continuous matrix product states. The advantage of this method is that it does not require introducing a cutoff, UV or IR, is fully non-perturbative, typically converges fast for Hamiltonians with polynomial interactions, and gives rigorous energy upper bounds. Its application to the Sinh-Gordon model is only partly successful: observables can be computed accurately up to fairly large values of the coupling, but the ultra strong coupling regime remains difficult to access without extrapolations. As a result, the behavior near the self-dual point is not yet fully settled. I will show the basics of relativistic continuous matrix product states, explain how they typically work for phi^4 like theories or even the easier Sine-Gordon model and finally discuss my attempts at taming Sinh-Gordon. | ||
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July 5, 2022 11:00 AM Rome Room 138 (SISSA) + virtual |
Antoine Tilloy |
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Attacking the Sinh-Gordon model with relativistic continuous matrix product states |
Non-reciprocal interactions are very common in natural systems. They can be used to explain the emergence of certain patterns such as bird flocking [1]. Generally, systems with non-reciprocal interactions are out of equilibrium, although they can obey detailed balance under certain conditions [2]. In this talk, I will present a particle model with non-reciprocal pair interactions between two species of drift-diffusive particles, say dogs and sheep. Following a path integral approach, I will discuss the stationary two-point correlation function and the entropy production. Even in the absence of drift, detailed balance is broken by non-reciprocity except for a particular choice of pair interactions. | ||
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June 21, 2022 11:00 AM Rome Virtual |
Rosalba Garcia Milan |
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Entropy Production of Non-reciprocal Interactions |
I consider nonequilibrium time evolution in quantum spin chains after a global quench. I show that global symmetries can invalidate the standard picture of local relaxation to maximum-entropy statistical ensembles and provide a solution to the problem. The issue arises when the Hamiltonian possesses conservation laws that are not (pseudo)local but act as such in the symmetry-restricted space where time evolution occurs. I focus on a specific example with a spin-flip symmetry and establish a connection with symmetry-protected topological order in equilibrium at zero temperature. In the second part of the talk I discuss some exceptional features of the infinite-time limit. In particular, the excess of entropy of a spin block triggered by a local perturbation in the initial state grows logarithmically with the subsystem’s length. Finally, I discuss the melting of the order induced either by a (symmetry-breaking) rotation of the initial state or by an increase of the temperature.
References: [1] M. Fagotti, Phys. Rev. Lett. 128, 110602 (2022) [arXiv:2110.11322] [2] M. Fagotti, V. Marić, and L. Zadnik, arXiv:2205.02221. |
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May 31, 2022 11:00 AM Rome Room 128 + virtual |
Maurizio Fagotti |
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Emergence of semilocal Gibbs ensembles |
Although basic equilibrium concepts like state variables, thermodynamic potentials, etc do not straightforwardly extend for non-equilibrium states, an analogue of Landau free energy characterising macroscopic fluctuations could be defined based on a mathematical theory of large deviations. I shall present a few old and new exact results for such a quantity in a class of non-equilibrium transport models. These are interacting classical many-body systems with relations to quantum spin chains. Our new results show that similar to a thermodynamic potential, these free-energy-analogues are robust against variations of coupling with external baths, thereby strengthening the possibility of a thermodynamic description for non-equilibrium states.
I shall explain how these exact results are derived using a tilted operator formalism and a matrix-product-representation for a specific integrable model [1]. Then, I shall recover these results using a minimal action solution of a hydrodynamic field theory [2], which applies to a wider class of systems, including those which are non-integrable. Refs. [1] Derrida, Hirschberg, Sadhu, 182 (2021) J Stat Phys; [2] in preparation. |
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May 24, 2022 11:00 AM Rome virtual |
Tridib Sadhu |
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Thermodynamic potential” for a class of non-equilibrium systems. |
I will give an overview of some developments about the long time dynamics of Hamiltonian partial differential equations, regarding the existence of periodic/quasi-periodic/almost periodic solutions, as well as long time Birkhoff normal form results, and chaotic dynamics and instability results. | ||
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May 17, 2022 11:00 AM Rome Room 128 + virtual |
Massimiliano Berti |
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Stable and Random motions in Hamiltonian PDEs |
Ever since its experimental observation, quantum many-body scarring as a weaker form of ergodicity breaking and ETH violation than the previously known integrability and many-body localization, has generated intensive theoretical efforts in searching exact excited states of models known to be non-integrable. Most of the examples found so far, fall in to the unified framework of spectrum generating algebra, sharing common features such as equal-distant energy spacing and area-law of entanglement entropy. In this talk, I present two distinct models, in one and two dimensions, with novel paradigms of allowing exact eigenstates or integrable subspectra. Their energy gaps and entanglement entropies of are also studies rigorously to show that neither fall into the categorial of conventional quantum many-body scars (QMBSs). These two models were constructed with two opposite approaches: the first model starts with a frustration-free bulk Hamiltonian and introduce boundary terms to split the ground state degeneracy. The second adds perturbation to an integrable Hamiltonian that violates Yang-Baxter equation in general, but leaves it satisfied only in certain subspaces of the Hilbert space, where the Bethe Ansatz eigenstates survive. These complementing strategies should help provide insight into the zoo of known QMBSs and formulating a general theory to guide their search in the future. | ||
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May 10, 2022 11:00 AM Rome Room 128 + virtual |
Zhao Zhang |
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Two toy models of novel quantum many-body scarring mechanisms |
The idea of describing properties of complicated physical systems using random matrices dates back to Wigner in the 1950s, originally in the context of heavy atomic nuclei. Since then, Random Matrix Theory has developed as an active branch of research at the interface between mathematics and physics, and is frequently used to diagnose whether a quantum many-body system behaves in a “regular” or “chaotic” way by comparing the statistics of its energy levels to theoretical predictions. The most commonly studied object is the statistics of the spacings between consecutive eigenvalues, but since the latter requires knowledge of the underlying density of states, which is non universal, it is often more practical to study the distribution of ratios between consecutive levels. Certain physical systems however possess some extra, sometimes hidden, symmetries, which result in a modification of the level statistics, somewhere between the “regular” and “chaotic” predictions. In this talk I will present how to extend the theory of spectral statistics to account for the presence of additional symmetries. More precisely, I will present surmises for the statistics of ratios when symmetries split the Hamiltonian into independent blocks, and discuss how these surmises can be used in practice. These provides a tool to not only get a signature of chaos or regularity in systems with symmetries, but also to uncover these symmetries if they were previously unnoticed. (based on O Giraud, N Macé, É Vernier, F Alet, Phys. Rev. X 12 (1), 011006) | ||
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May 3, 2022 11:00 AM (Rome) Room 005 + virtual |
Eric Vernier |
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Probing symmetries of quantum many-body systems through spectral statistics |
This talk is about the computation of the relative entropy associated to an interval between a thermal state and a coherent excitation of itself in the bosonic U(1)-current model, namely the (derivative of the) chiral boson. The main goal is to compute the corresponding relative entropy of Araki. For this purpose I will start by presenting some well-known facts on the algebra of Canonical Commutation Relations as well as more recent results of R. Longo and collaborators on the entropy of subspaces and of Bostelmann, Cadamuro and Del Vecchio on the relative entropy of non-pure states (by purification) such as thermal states. Then, by extending previous results of Borchers and Yngvason (which will be briefly recalled) it will be possible to make use of the full PSL(2,R) symmetries and hopefully become clear how to arrive at the sought relative entropy. Such entropy entails both a Bekenstein-like bound and a QNEC-like bound, which deserve further investigation. The talk is based on work in progress with Gabriel Palau. | ||
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April 26, 2022 11:00 AM (Rome) Room 128 + virtual |
Alan Garbarz |
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Relative entropy of an interval for the chiral boson at finite temperature |
The recent advances in machine learning algorithms have boosted the application of these techniques to the field of condensed matter physics. Here we demonstrate that, if we employ a training set made by single-particle correlation functions of a non-interacting quantum wire,unsupervised and supervised machine learning techniques are able to reconstruct the phase diagram of a related interacting model and identify topological phases with a high degree of accuracy. |
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April 12, 2022 11.00 (Rome) Virtual + Room 128 (SISSA) |
Elisa Ercolessi |
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Detection of topological phases of interacting models from single-particle correlation functions via machine learning |
As phenomena that necessarily emerge from the collective behavior of interacting particles, phase transitions continue to be difficult to predict using statistical thermodynamics. A recent proposal called the topological hypothesis suggests that the existence of a phase transition could perhaps be inferred from changes to the topology of the accessible part of the configuration space. We instead suggest that such a topological change is often associated with a dramatic change in the configuration space geometry, and that the geometric change is the actual driver of the phase transition. More precisely, a geometric change that brings about a discontinuity in the mixing time required for an initial probability distribution on the configuration space to reach steady-state is conjectured to be related to the onset of a phase transition in the thermodynamic limit. This conjecture is tested by evaluating the diffusion diameter and epsilon-mixing time of the configuration spaces of hard disk and hard sphere systems of increasing size. Explicit geometries are constructed for the configuration spaces of these systems, and numerical evidence suggests that a discontinuity in the epsilon-mixing time coincides with the solid-fluid phase transition in the thermodynamic limit. | ||
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April 5, 2022 17.00 (Rome) virtual |
Ozan B. Ericok |
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A geometric conjecture about phase transitions |
I will review the role of universality both at equilibrium and out of equilibrium studied with tensor networks. In particular I will discuss some features of universal entanglement spectra for Ising and Potts model in 1+1D, and a recently discovered critical point in the Abelian Higgs model in 1+1D. | ||
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March 29, 2022 11.00 (Rome) Room 128 – SISSA and virtual |
Luca Tagliacozzo |
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Quest for criticality at an out of equilibrium with tensor network states |
The jamming transition is a zero-temperature phenomenon related to disordered and glassy systems. Its critical behaviour has been characterized in the solution of the hard sphere model in infinite spatial dimensions, corresponding to a mean-field theory. In this talk, we consider soft-spheres with a linear repulsive potential and their mean-field model, i.e. the perceptron. We show that the jamming critical behaviour gets extended from the jamming point to an entire phase. We characterize this self-organized critical and marginally stable phase and we show the emergence of a symmetry in the critical exponents. In the last part of the talk, we will discuss jamming criticality in optimization problems and machine learning. | ||
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March 22, 2022 11.00 (Rome) Room 128 – SISSA and virtual |
Antonio Sclocchi |
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A new critical phase in jammed models |
Systems of active matter, such as bacterial colonies and cell monolayers, show intriguing patterns in their velocity field, displaying spatial velocity correlations. In this talk, I show that spherical Active Brownian Particles (ABP) reproduce these phenomena because of the interplay between persistent active forces and pure repulsive interactions, without invoking explicit alignment forces. Both in phase-separated and homogeneous configurations, ABPs display coherent domains where the particle velocities are spontaneously aligned and spatial velocity correlations are observed. These spontaneous collective effects have a non-thermal origin, being independent of active and solvent temperatures, while their dynamical nature is corroborated by a strong dependence on the active force persistence and inertial time. We build a phase diagram as a function of packing fraction and persistence time to compare the structural properties of the system (i.e. liquid, hexatic and solid phases) with the emergent velocity order (i.e. the correlation length of the spatial velocity correlations). For active solids, our findings are corroborated by a microscopic theory while, for active liquid, we developed a hydrodynamic theory, derived by the microscopic model under suitable approximations. In the liquid case, the spatial structure and the slowdown of the velocity field are shown through the analytical expressions for the dynamical correlation functions. | ||
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March 15, 2022 11.00 am (Rome) Virtual |
Lorenzo Caprini |
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Spatial velocity correlations and spontaneous velocity order in active Brownian particles |
In recent years, there has been intense attention on the constraints imposed by quantum mechanics on the dynamics of many-body systems at low temperatures, triggered by the postulation and derivation of quantum bounds on transport coefficients or on the chaos rate. In this talk I will discuss the quantum fluctuation-dissipation theorem (the KMS conditions) as the principle underlying bounds on correlation time scales. By restating the problem in a replicated space, we show that the quantum bound to chaos is in fact a direct consequence of the KMS condition, as applied to a particular pair of two-time correlation and response functions. Encouraged by this, we describe how quantum fluctuation-dissipation relations act in general as a blurring of the time-dependence of correlations, which can imply bounds on their decay rates. Thinking in terms of fluctuation-dissipation opens a direct connection between bounds and other thermodynamic properties.
S. Pappalardi, L. Foini, J. Kurchan, arXiv:2110.03497 |
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March 1, 2022 11.00 am (Rome) Room 128 – SISSA and virtual |
Laura Foini |
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Quantum bounds and fluctuation-dissipation relations |
The last decades have seen groundbreaking progress in non-perturbative aspects of quantum field theory (QFT) using methods from quantum information theory. Most of these results have been so far restricted to relativistic QFT because constraints from Lorentz symmetry and causality play a key role in the above approaches. Much less is known in non-relativistic QFT, in particular, in the continuum QFT itself. In this talk, I will present the results of the entanglement entropies of an interval on the infinite line for a free fermionic spinless Schrödinger field theory at finite density, which is a non-relativistic model with Lifshitz exponent z=2. In this case, the entanglement spectrum can be exactly obtained and expressed in terms of prolate spheroidal wave functions. Analogously, the entropies can be also studied by means of the Fredholm determinant of the two-point correlator, which turns out to be the so-called tau function of a Painlevé V differential equation and it can be expressed in terms of Virasoro irregular conformal blocks of c=1. I will also discuss generalizations of these results to a class of free fermionic Lifshitz models labeled by integer dynamical exponent z. | ||
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Feb 22, 2022 11.00 am (Rome) Room 128 – SISSA and virtual |
Diego Pontello |
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Entanglement entropies in free Schrödinger field theory at finite density |
The understanding of the relaxation of large quantum systems has received an important boost from the point of view of pure state quantum statistical mechanics, and in particular from the eigenstate thermalization hypothesis. At the same time, the dynamics of quantum systems has been investigated by out of time ordered correlators. Interestingly it was shown, using hydrodynamic theory, that such correlators relax algebraically in the presence of conserved quantities. Here we show how such slow relaxation can be expected from the eigenstate thermalization hypothesis. At the same time, conserved quantities can lead to the phenomenon of pre-thermalization. Here we show that considering two large non-integrable systems weakly coupled to each other, one can show that in the thermodynamic limit a steady current between the two will emerge, and that this current is typical. |
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Feb 15, 2022 11.00 am (Rome) Virtual and in presence at Budinich Lecture Hall (ICTP) |
Dario Poletti |
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From conserved quantities to the emergence of slow relaxation and steady currents |
The complex world surrounding us, including all living matter and various artificial complex systems, is mostly far from thermal equilibrium. A major goal of modern statistical physics and thermodynamics is to unravel the fundamental principles governing the behaviour of such nonequilibrium systems, like the swarming of fish, flocking of birds, or pedestrian crowd dynamics. An important novel concept to describe and classify nonequilibrium systems is the stochastic entropy production, which explicitly quantifies the breaking of time-reversal symmetry. However, so far, little attention has been paid to the implications of non-conservative interactions, such as time-delayed (i.e., retarded) or nonreciprocal interactions, which cannot be represented by Hamiltonians contrasting all interactions traditionally considered in statistical physics. Such interactions indeed emerge commonly in biological, chemical and feedback systems, and are widespread in engineering and machine learning. In this talk, I will use simple time- and space-continuous models to discuss technical challenges and unexpected physical phenomena induced by nonreciprocity [1,2] and time delay [3,4].
[1] Loos and Klapp, NJP 22, 123051 (2020) |
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Feb 01, 2022 11.00 am (Rome) Virtual |
Sarah Loos |
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The role of non-conservative interactions in nonequilibrium complex systems |
The time evolution of entanglement in nonequilibrium quantum many-body systems provides a lot of information about the underlying dynamics. Among others, studies of entanglement spreading from highly excited initial states of integrable systems have contributed a lot to our understanding of quasiparticle dynamics. In this talk I will consider the case of low energy excitations above the ground state, created by a local operator,and show that the growth of entanglement can still be characterized by a quasiparticle picture. The case of extended excitations will also be discussed, which interpolates between local and global quench scenarios. | ||
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January 25, 2022 11.00 (rome) Virtual |
Viktor Eisler |
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Entanglement spreading after local and extended excitations |
Learning how to create, study, and manipulate highly entangled states of matter is key to understanding exotic phenomena in condensed matter and high energy physics, as well as to the development of useful quantum computers. In this talk I will discuss recent experiments where we demonstrated the realization of a quantum spin liquid phase using Rydberg atoms on frustrated lattices and a new architecture based on the coherent transport of entangled atoms through a 2D array. Combining these results with novel technical tools on atom array platforms could open a broad range of possibilities for the exploration of entangled matter, with powerful applications in quantum simulation and information. | ||
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Jan 18, 2022 14:00 PM Rome Virtual |
Giulia Semeghini |
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New frontiers in quantum simulation and computation with neutral atom arrays |
The concept of causality, stating that physical actions cannot propagate in space at an arbitrary speed, can be captured for qudit systems by the notion of Quantum Cellular Automata (QCA), defined as unitary maps preserving locality of observables. In this talk, I will show that QCA can be identified, in any dimension and geometry, with special tensor network operators, yielding a general connection between causality and bounds on entanglement production in the form of area laws. I will stress the importance of unitarity, by discussing generalizations of our results for different classes of non-unitary quantum channels. Finally, I will show how the set of QCA can be extended to a larger class of deterministic maps via LOCC (local operations and classical communication) and illustrate implications on state-preparation protocols and classification of phases of matter.
Talk based on: [2] LP, G. Styliaris, and J. I. Cirac, Quantum Circuits Assisted by LOCC: Transformations and Phases of Matter, |
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Jan 11, 2022 11:00 AM Rome Virtual |
Lorenzo Piroli |
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Quantum Cellular Automata, Tensor Networks, and Area Laws |
2021 seminars |
When a generic isolated quantum many-body system is driven out of equilibrium, its local properties are eventually described by the thermal ensemble. This picture can be intuitively explained by saying that, in the thermodynamic limit, the system acts as a bath for its own local subsystems. Despite the undeniable success of this paradigm, for interacting systems most of the evidence in support of it comes from numerical computations in relatively small systems, and there are very few exact results. In the talk, I will present an exact solution for the thermalization dynamics in the “Rule 54″ cellular automaton, which can be considered the simplest interacting integrable model. After introducing the model and its tensor-network formulation, I will present the main tool of my analysis: the space-like formulation of the dynamics. Namely, I will recast the time-evolution of finite subsystems in terms of a transfer matrix in space and construct its fixed-points. This construction provides the full description of subsystem dynamics and enables us to fully characterize relaxation time-scales. I will conclude by discussing the growth of entanglement after the quench, and show the agreement with the quasi-particle picture. The talk is based on a recent series of papers: arXiv:2012.12256, arXiv:2104.04511, and arXiv:2104.04513. | ||
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Dec 21, 2021 11:00 AM Rome Virtual |
Katja Klobas |
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Exact description of quench dynamics and entanglement spreading in Rule 54 |
Distinguishing genuine quantum correlation (entanglement) from spurious statistical one in out-of-equilibrium open quantum many-body systems is in general a challenging task. In this talk I will review some recent efforts to describe the entanglement dynamics in the presence of Markovian dissipation. In particular, I will focus on free-fermion and free-bosons subjected to global linear dissipation as well as to localized one. I will show that, at least for free systems, it is possible to incorporate the effects of dissipation in a hydrodynamic description for the entanglement spreading (quasiparticle picture). | ||
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Dec 16, 2021 2:00 PM Rome Virtual |
Vincenzo Alba |
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Entanglement dynamics and dissipation in out-of-equilibrium systems |
Two dimensional gases of non intersecting loops have been a subject of study in mathematical physics for more than thirty years because of their numerous connections to integrability, two dimensional conformal field theory, random geometry and combinatorics. In this talk, I will present a natural generalization of loop models to gases of graphs possessing branchings. These graphs are called webs and first appeared in the mathematical community as diagrammatic presentations of categories of representations of quantum groups. The web models posses properties similar to the loop models. For instance, it will be shown that they describe, for some tuning of the parameters, interfaces of spin clusters in Zn spin models. Focusing on the numerically more accesible case of Uq(sl3) webs (or Kuperberg webs), it is possible to identify critical phases that are analogous to the dense and dilute phases of the loop models. These phases are then described by a Coulomb Gas with a two component bosonic field. | ||
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Dec 7, 2021 11:00 AM Rome Room 134. SISSA (via Bonomea 265) |
Augustin Lafay |
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Web models as generalizations of statistical loop models. |
In this talk I will present a novel scheme for the exact (aka functional) renormalisation group motivated by the desire of reducing the complexity of practical computations. The key idea is to specify renormalisation conditions for all inessential couplings, leaving one with the task of computing only the flow of the essential ones. To achieve this aim, a renormalisation group equation for the effective average action which incorporates general non-linear field reparameterisations is utilised. A prominent feature of the scheme is that, apart from the renormalisation of the mass, the propagator evaluated at any constant value of the field maintains its unrenormalised form. Conceptually, the scheme provides a clearer picture of renormalisation itself since the redundant, non-physical content is automatically disregarded in favour of a description based only on quantities that enter expressions for physical observables. To exemplify the scheme’s utility, I will present the investigation of the Wilson-Fisher fixed point in three dimensions at order two in the derivative expansion. In this case, the scheme removes all terms with two derivatives apart from the canonical term. Further simplifications occur at higher orders in the derivative expansion. | ||
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November 30, 2021 11.00 AM Rome Room 128. SISSA (via Bonomea 265) |
Kevin Falls |
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Essential renormalisation group |
The Kondo effect is an archetype of quantum many-body physics: a magnetic impurity coupled antiferromagnetically to a metallic bath results in a strongly correlated ground state in which the bath screens the impurity spin. If the bath is superconducting rather than metallic, a so-called Yu-Shiba-Rusinov (YSR) bound state forms which screens the impurity and whose energy is below the superconducting gap. These YSR states have recently garnered a lot of interest due to applications in topological quantum computing but are typically only studied for BCS superconductors. In this talk I will describe how this BCS picture changes when a Kondo impurity is coupled to a 1-d charge conserving superconductor. By examining the system in various semiclassical limits through bosonization, combined with exact results from Bethe Ansatz the full phase diagram can be determined. The main result is that the enhanced quantum fluctuations lead to the destruction of YSR states in all but a very narrow region of the phase diagram. Within this region, the YSR state facilitates a first order quantum phase transition between a screened and unscreened impurity. Outside this region a renormalized Kondo effect is found on the screened side whereas on the unscreened side the impurity remains strongly coupled to the bulk in contrast to the metallic case. | ||
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November 23, 2021 11.00 (Rome) Room 128, SISSA (via Bonomea 265) |
Colin Rylands |
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The Kondo effect in a 1-d charge conserving superconductor |
Conformal field theories (CFTs) are ubiquitous in theoretical physics as fixed points of renormalization, descriptions of critical systems and more. In these theories the conformal symmetry is a powerful tool in the computation of correlation functions, especially in 2 dimensions where the conformal algebra is infinite. Discretization of field theories is another powerful tool, where the theory on the lattice is both mathematically well-defined and easy to put on a computer. In this talk I will outline how these are combined using a discrete version of the 2d conformal algebra that acts in lattice models. I will also discuss recent work on convergence of this discretization, as well as on applications to non-unitary CFTs that appear in descriptions of problems of interest in condensed matter physics such as polymers, percolation and disordered systems. | ||
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Nov 9, 2021 11.00 Room 128 |
Anna-Linnea Grans-Samuelsson |
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“Discretizing 2d conformal field theories: the lattice action of the conformal algebra” |
Quantum entanglement of pure states has led to new insights into a wide variety of topics. Entanglement of mixed states is however less well understood. In this talk I will focus on a few themes where mixed-state entanglement leads to new insights that are difficult to obtain otherwise. I will mainly focus on two topics: (i) Characterizing finite-temperature topological order [1] and classicality/quantum-ness of phase transitions [2] (ii) Detecting presence of quasiparticles and quantum chaos [3].
Work done in collaboration with Tsung-Cheng Lu, Tim Hsieh, Kai-Hsin Wu, Chia-Min Chung and Ying-Jer Kao. [1] Phys. Rev. Lett. 125, 116801 (2020). |
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June 22, 2021 5.00 PM Rome Virtual Seminar |
Tarun Grover |
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Mixed-state entanglement as a probe of topological order, quantum phase transitions and quasiparticles |
Over the last few years it was realised that it is possible to understand string theory and the AdS/CFT correspondence by borrowing ideas techniques from the theory of integrable models. This lead to remarkable advances in AdS/CFT as well as in the study of integrable S matrices and form factors. In this talk I plan to give a pedagogical introduction to these advances. The talk does not require any advanced knowledge of string theory or of integrability; questions and discussions are highly encouraged. | ||
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Jun 8, 2021, 2021 11:00 AM Rome Virtual Seminar |
Alessandro Sfondrini |
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New integrable models to understand string theory |
Defining the chaotic properties of quantum systems is a notoriously difficult problem, because of the fundamental fact that in quantum mechanics trajectories are not well-defined. A historically very productive direction has been the investigation of spectral properties of quantum Hamiltonian in a statistical way: it emerges that chaos is associated with strong repulsion between energy levels. When one tries to apply this recipe to many-body systems, it is clear that a hierarchy of energy/time scales can emerge due to the interplay of farther components of the system which are less and less correlated. Recently, the use of quantum circuits composed by random gates has allowed explicit calculations clarifying to what extent chaos emerges for spatially extended systems. In this talk, I will review some recent advances in the field with an emphasis on the strong success of a peculiar symmetry which exchanges space and time. Thanks to this formulation, we have been able to connect spectral properties to a spectrum of Lyapunov exponents, which emerge from the infinite product of random operators in the space direction. | ||
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May 18, 2021 11:00 AM Virtual Seminar |
Andrea De Luca |
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Spectral statistics in the thermodynamic limit of extended many-body quantum systems |
We introduce a Metropolis-Hastings Markov chain for Boltzmann distributions of classical spin systems. It relies on approximate tensor network contractions to propose correlated collective updates at each step of the evolution. We present benchmarks for a wide variety of instances of the two-dimensional Ising model, including ferromagnetic, antiferromagnetic, (fully) frustrated and Edwards-Anderson spin glass cases, and we show that, with modest computational effort, our Markov chain achieves sizeable acceptance rates, even in the vicinity of critical points. In each of the situations we have considered, the Markov chain compares well with other Monte Carlo schemes such as the Metropolis or Wolff algorithm: equilibration times appear to be reduced by a factor that varies between 40 and 2000, depending on the model and the observable being monitored. We also present an extension to three spatial dimensions, and demonstrate that it exhibits fast equilibration for finite ferro and antiferromagnetic instances. Additionally, and although it is originally designed for a square lattice of finite degrees of freedom with open boundary conditions, the proposed scheme can be used as such, or with slight modifications, to study triangular lattices, systems with continuous degrees of freedom, matrix models, a confined gas of hard spheres, or to deal with arbitrary boundary conditions. Joint work with Miguel Frías-Pérez, Michael Mariën, David Pérez García, and Mari Carmen Bañuls (arXiv:2104.13264). | ||
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Tuesday, May 11, 2021 11.00 AM Virtual Seminar |
Sofyan Iblisdir |
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Collective Monte Carlo updates through tensor network renormalization |
High-dimensional random functionals emerge ubiquitously when modeling the energy landscapes of complex systems, and are typically glassy: exploring them with stochastic dynamics is highly non-trivial due to the abundance of metastable minima that trap the system for very large times. The resulting slow dynamics is dominated by activated processes, in which the system jumps between local minima passing through the saddles (or transition states) connecting them. These jumps can be thought of as instantons of an associated dynamical theory. In simple cases (such as in double-well potentials in 1d), instantons can be constructed using the information on the two energy minima and the barrier between them. In high-dimension, the proliferation of minima renders the problem much more complicated: which other minima does the system reach once it escapes from a particular metastable state? Which are the saddles involved in the associated transition path? In this talk I will focus on random Gaussian functionals in high-dimension, for which these questions can be addressed statistically. I will discuss how to use random matrix theory to gain information on the distribution and reciprocal arrangement of the local minima and saddles, and how to exploit this information to build simple dynamical instantons describing activated jumps between nearby minima in configuration space. | ||
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Tuesday, May 4, 2021 11.00 AM Rome Virtual Seminar |
Valentina Ros |
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Dynamics in random glassy landscapes: towards high-dimensional instantons |
I will discuss exactly-solvable models of dynamics for some classes of isolated and dissipative Floquet-driven quantum systems. | ||
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Apr 27th, 2021 11:00 AM Rome Virtual Seminar |
Vladimir Gritsev |
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Integrable Floquet dynamics of many-body quantum systems |
Recently, the study of chaos in quantum systems has been revitalized due to what is now known as the bound to chaos. This result limits the rate of growth of chaos at low temperatures due to quantum effects.
In this talk, I will present ongoing work with Jorge Kurchan concerning the bound in the context of classical and quantum free dynamics on curved manifolds. Thanks to the curvature, such models display chaotic dynamics up to low temperatures, due to the absence of any localizing potentia |
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Apr 20th, 2021 11:00 AM Rome Virtual Seminar |
Silvia Pappalardi |
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Low temperature chaos and the quantum effects of curvature |
In this talk I will review the main results and ideas of two recent papers [arXiv:2005.11266, arXiv:2103.03735]. In these papers we studied an integrable quantum field theory (IQFT) using the generalized hydrodynamics approach for different initial state configurations (equilibrium, partitioning protocol, gaussian temperature profile). The model we considered has two special features: it allows for the formation of an unstable particle from the scattering of two stables ones, and its scattering matrix breaks parity invariance. By studying typical hydrodynamic quantities such as the effective velocities of stable particles and their densities, we have found that they exhibit unique features that may be interpreted as universal signatures of the formation and decay of unstable particles. These signatures allow us to “visualize” the dynamics of unstable particles in IQFT hence, to give an interesting physical/phenomenological interpretation that is absent when they are merely associated with poles of the scattering matrix. | ||
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Apr 13, 2021, 2021 11:00 AM Virtual Seminar |
Olalla Castro-Alvaredo |
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Unstable Particles and their Signatures from Generalized Hydrodynamics |
Many-body localization is a fascinating theoretical concept describing the intricate interplay of quantum interference, i.e., localization, with many-body interaction-induced dephasing. While numerous computational tests and also several experiments have been put forward to reveal the basic concepts, the overall understanding of the phenomenon is still limit; an important contributing factor is the lack of a microscopic analytical theory.
In this talk we will survey the status and recent progress in numerical simulations of the charge dynamics in the ergodic and non-ergodic regimes of disorder. A particular emphasis will be on the long-time asymptotics of temporal phenomena in wires of a finite length: they reveal a plethora of phenomena, such as hypersensitivity to the finite system size and manifestatitions of multifractality in return probabilities and dephasing times. The talk is based on the publications: F. Weiner, FE, and S. Bera, Slow dynamics and strongfinite-size effects in many-body localization with random andquasiperiodic potentials, Phys.Rev.B 100, 104204 (2019). S. Bera, G. De Tomasi, F. Weiner, and FE, DensityPropagator for Many-Body Localization: Finite-Size Effects,Transient Subdiffusion, and Exponential Decay, Phys. Rev.Lett. 118, 196801 (2017). |
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Tuesday, Mar 30, 2021 11:00 Virtual Seminar |
Ferdinand Evers |
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Dynamics and dephasing in strongly disordered interacting quantum wires |
The ε expansion was invented almost 50 years ago and has been used extensively ever since to study aspects of renormalization group flows and critical phenomena. Its most famous applications are found in theories involving scalar fields in 4−ε dimensions. In this talk, we will discuss the structure of the ε expansion in scalar field theories and the fixed points that can be obtained within it. Our motivation is based on the goal of classifying conformal field theories in d=3 dimensions. We will describe recently discovered universal constraints obtained within the framework of the ε expansion, focusing mostly on the 4−ε case although 3−ε will also be discussed. It will be shown that a “heavy handed” way to search for fixed points yields a plethora of new fixed points that reveal aspects of the structure of the ε expansion and suggest that a classification of conformal field theories in d=3 is likely to be highly non-trivial. | ||
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Tuesday, Mar 16, 2021 17.00 Virtual Seminar |
Andreas Stergiou |
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Uncovering the Structure of the ε Expansion |
Exceptional points (EPs) are ubiquitous in non-hermitian systems, and represent the complex counterpart of critical points. By driving a system through a critical point at finite rate induces defects, described by the Kibble-Zurek mechanism, which finds applications in diverse fields of physics. Here we generalize this to a ramp across an EP and demonstrate that for a variety of drives, the defect density scales as v^[âˆ’(d+z)Î½/(zÎ½+1]) in terms of the usual critical exponents and v the speed of the drive. Defect production is suppressed compared to the conventional hermitian case as the defect state can decay back to the ground state close to the EP. By using single-photon interferometry, we also reconstruct the above scaling experimentally. | ||
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Tuesday, Mar 09, 2021 11h 00 Virtual Seminar |
Balazs Dora |
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The Kibble-Zurek mechanism at exceptional points |
Tensor network techniques have been crucial to study strongly coupled spin systems. The application of these techniques to quantum fields presents the challenge of dealing with infinite dimensional systems. Inspired on standard field theory techniques, we propose a new strategy based on preserving the continuum character of fields and use them as building blocks of an adapted Tensor Renormalization Group protocol. Its viability is tested by evaluating the partition function of a free massive boson on a 2D square lattice and a phi^4 theory at first order in the coupling constant. The Boltzmann weights of the free lattice model are shown to satisfy the Yang-Baxter equation with a uniformization given by trigonometric functions in the massless case, and Jacobi elliptic functions in the massive case. A related factorizable S-matrix model for continuous degrees of freedom is constructed. These results place the free boson in 2D in the same position as all other models that are exactly solvable `a la Yang-Baxter. | ||
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Mar 2, 2021 11:00 Virtual Seminar |
Esperanza Lopez |
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A tensor network for quantum fields |
Entanglement phase transitions in quantum chaotic systems subject to projective measurements or in random tensor networks have emerged as a new class of critical points separating “phases” with different entanglement scaling. In this talk, I will propose a theoretical framework for studying the universal properties of such transitions, using a mapping onto replica statistical mechanics models. I will discuss consequences for the critical properties of entanglement dynamics near entanglement transitions, and focus on various solvable limits. | ||
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February 23, 2021 14:00 Virtual Seminar |
Romain Vasseur |
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Entanglement transitions |
Identifying the relevant coarse-grained degrees of freedom in a complex physical system is a key stage in developing effective theories. The celebrated renormalization group (RG) provides a framework for this task, but its practical execution in unfamiliar systems is fraught with ad hoc choices. Machine learning approaches, on the other hand, though promising, often lack formal interpretability: it is unclear what relation, if any, the architecture- and training-dependent learned “relevant” features bear to standard objects of physical theory. I will present recent results addressing both issues. We develop a fast algorithm, the RSMI-NE, employing state-of-art results in machine-learning-based estimation of information-theoretic quantities to construct the optimal coarse-graining. We use it to develop a new approach to identifying the most relevant field theory operators describing a statistical system, which we validate on the example of interacting dimer model. I will also discuss the formal results underlying our methods: we establish equivalence between the information-theoretic notion of relevance defined in the Information Bottleneck (IB) formalism of compression theory, and the field-theoretic relevance of the RG. We show analytically that for statistical physical systems the “relevant” degrees of freedom found using IB compression (and RSMI-NE) indeed correspond to operators with the lowest scaling dimensions. Our findings provide a dictionary connecting two distinct theoretical toolboxes, and conceptually pave the way towards automated theory-building. |
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February 2, 2021 11:00 Virtual Seminar |
Maciej Koch-Janusz |
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Statistical physics through the lens of real-space mutual information |
The percolation properties of random fields arise naturally in many contexts, ranging from planet science to transport in disordered systems. In this talk we will discuss new results concerning the percolation transition of long-range correlated random fields. In particular we show that the level sets of surfaces with negative Hurst exponent are conformal fractals. Moreover, we revisit and solve the long-standing problem of the percolation transition in the 2D Gaussian Free Field (GFF). We show the existence of a non-trivial transition where the level set of the GFF become “logarithmic fractals”. The correlation function of such fractals is also exactly computed. |
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January 26, 2021 11:00 Virtual Seminar |
Raoul Santachiara |
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The topography of random surfaces: percolation transitions and conformal fractals |
Weyl semimetals are 3D condensed matter systems characterized by a degenerate Fermi surface, consisting of a pair of `Weyl nodes’. Correspondingly, in the infrared limit, these systems behave effectively as Weyl fermions in 3+1 dimensions. We consider a class of interacting 3D lattice models for Weyl semimetals and prove that the quadratic response of the quasi-particle flow between the Weyl nodes, which is the condensed matter analogue of the chiral anomaly in QED4, is universal, that is, independent of the interaction strength and form. Universality, which is the counterpart of the Adler-Bardeen non-renormalization property of the chiral anomaly for the infrared emergent description, is proved to hold at a non-perturbative level, notwithstanding the presence of a lattice (in contrast with the original Adler-Bardeen theorem, which is perturbative and requires relativistic invariance to hold). The proof relies on constructive bounds for the Euclidean ground state correlation functions combined with lattice Ward Identities, and it is valid arbitrarily close to the critical point where the Weyl points merge and the relativistic description breaks down. Joint work with V. Mastropietro and M. Porta. | ||
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January 19, 2021 11:00 Virtual Seminar |
Alessandro Giuliani |
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Non-renormalization of the `chiral anomaly’ in interacting lattice Weyl semimetals |
Generic short-range interacting quantum systems with a conserved quantity exhibit universal diffusive transport at late times. We show how this universality is replaced by a more general superdiffusive transport process in the presence of long-range interactions, decaying algebraically with distance. While diffusive behavior is recovered for a sufficiently fast decay, longer-ranged couplings give rise to effective classical Levy flights; a random walk with step sizes following a heavy-tailed distribution (i.e. falling off algebraically at large distances). We study this phenomenon in a long-range interacting XY spin chain with conserved total magnetization, at infinite temperature. We investigate the dynamics by employing non-equilibrium quantum field theory and semi-classical phase space simulations. We find that the space-time dependent spin density profiles are self-similar, and show superdiffusive spreading, with scaling functions given by the stable symmetric distributions. We also extract the associated generalized diffusion constant, and demonstrate that it follows the prediction of classical Levy flights; quantum many-body effects manifest themselves in an overall time scale depending only weakly on the precise form of the algebraic long-range interaction. Our findings can be readily verified with current trapped ion experiments. | ||
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January 12, 2021 11:00 Virtual Seminar |
Izabella Lovas |
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Non-local emergent hydrodynamics in a long-range quantum spin system |
2020 seminars |
Over the past few decades, there have been spectacular experimental developments in manipulating cold atoms (bosons or fermions) [1, 2], which allow one to probe quantum many-body physics, both for interacting and noninteracting systems. In this talk we focus on the noninteracting Fermi gas, for which a general theoretical framework has been developed over the recent years [3,4]. We consider a generic model of N non-interacting spinless fermions in d dimensions confined by a general trapping potential (we assume a central potential for d>1), in the ground-state. In d=1, for specific potentials, this system is related to classical random matrix ensembles… |
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Dec 15, 2020 11:00 Virtual Seminar |
Naftali R. Smith |
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Counting statistics for non-interacting fermions in a d-dimensional potential |
The connection between statistical mechanics and the combinatorics of alternating sign matrices is known since the work of Razumov and Stroganov on the spin-1/2 XXZ chain. One important example of this combinatorial relation occurs in the study of the emptiness formation probability EFP(N,m). This observable is defined as the sum of the squares of the ground state components of the Hamiltonian for the chain of length N, restricted to components where m consecutive spins are aligned. At the combinatorial point Δ = -1/2, it takes the form of a simple product of integers. This was shown by Cantini in 2012.
In this talk, I discuss joint work with C. Hagendorf and L. Cantini where we define a new family of overlaps C(N,m) for the spin-1/2 XXZ chain. It is equal to the linear sum of the groundstate components that have m consecutive aligned spins. For reasons that will be discussed, we refer to the ratio C(N,m)/C(N,0) as the boundary emptiness formation probability. We compute C(N,m) at the combinatorial point as a simple product of integers. |
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December 10, 2020 14:00 Virtual Seminar |
Alexi Morin-Duchesne |
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Boundary emptiness formation probabilities in the six-vertex model at Δ = -1/2 |
In this talk I will discuss some recent developments in the study of entanglement in 1+1 and 2+0 dimensions quantum field theories. The seminar is divided into two parts, which are self-contained. In the first half I will review results for entanglement dynamics obtained in the scaling limit of the Ising spin chain, by using a form factor perturbation theory. Speculations about thermalization at large times and its absence will be given; this is a published joint work with O. Castro-Alvaredo, M. Lencses and I. Szecsenyi. In the second part, I will explain how genus two partition functions in CFTs with c<1 can be calculated using the conformal bootstrap. Such partition functions appear in the study of the mutual information and negativity and so-far there was no general technique to determine them. If not already in zeitnot, I will discuss two examples: the Lee Yang and the Ising CFT. The second part is an ongoing collaboration with F. Ares. |
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December 1, 2020 11:00 Virtual Seminar |
Jacopo Viti |
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Entanglement entropies and the modular bootstrap for Z_3 Riemann surfaces |
Realizing strongly-correlated topological phases of ultracold gases is a central goal for ongoing experiments. And while fractional quantum Hall states could soon be implemented in small atomic ensembles, detecting their signatures in few-particle settings remains a fundamental challenge. In this work, we numerically analyze the center-of-mass Hall drift of a small ensemble of hardcore bosons, initially prepared in the ground state of the Harper-Hofstadter-Hubbard model in a box potential. By monitoring the Hall drift upon release, for a wide range of magnetic flux values, we identify an emergent Hall plateau compatible with a fractional Chern insulator state: the extracted Hall conductivity approaches a fractional value determined by the many-body Chern number, while the width of the plateau agrees with the spectral and topological properties of the prepared ground state. Besides, a direct application of Streda’s formula indicates that such Hall plateaus can also be directly obtained from static density-profile measurements. Our calculations suggest that fractional Chern insulators can be detected in cold-atom experiments, using available detection methods. |
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November 24, 2020 11:00 Virtual Seminar |
Cécile Repellin |
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Detecting fractional Chern insulators in few-boson systems |
We consider an active run-and-tumble particle (RTP) in arbitrary dimension d and compute exactly the probability S(t) that the x-component of the position of the RTP does not change sign up to time t. For the most relevant case of an exponential distribution of times between consecutive tumblings, we show that S(t) is independent of d for any finite time t, as a consequence of the celebrated Sparre Andersen theorem for discrete-time random walks in one dimension. Moreover, we show that this universal result holds for a much wider class of RTP models in which the velocity v of the particle after each tumbling is drawn randomly from an arbitrary probability distribution. | ||
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November 17, 2020 11.00 Virtual Seminar |
Satya Majumdar |
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Universal survival probability for a d-dimensional run-and-tumble particle |
We consider the entanglement entropies of energy eigenstates in quantum many-body systems. For the typical models that allow for a field-theoretical description of the long-range physics, we find that the entanglement entropy of (almost) all eigenstates is described by a single scaling function. This is predicated on the applicability of the weak or strong eigenstate thermalization hypothesis (ETH), which then implies that the scaling functions can be deduced from subsystem entropies of thermal ensembles. The scaling functions describe the full crossover from the ground state entanglement regime for low energies and small subsystem size (area or log-area law) to the extensive volume-law regime for high energies or large subsystem size. For critical 1d systems, the scaling function follows from conformal field theory (CFT). We use it to also deduce the scaling function for Fermi liquids in d>1 dimensions. These analytical results are complemented by numerics for large non-interacting systems of fermions in d=1,2,3 and the harmonic lattice model (free scalar field theory) in d=1,2. Lastly, we demonstrate ETH for entanglement entropies and the validity of the scaling arguments in integrable and non-integrable interacting spin chains. In particular, we analyze the XXZ and transverse-field Ising models with and without next-nearest-neighbor interactions.
References: arXiv:1905.07760, arXiv:1912.10045, arXiv:2010.07265 |
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November 10, 2020 11:00 Virtual Seminar |
Thomas Barthel |
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Entanglement entropy of energy eigenstates follows a universal scaling function |
Controlling the spread of correlations in quantum many-body systems is a key challenge at the heart of quantum science and technology. Desired correlations are usually destroyed by dissipation arising from coupling between a system and its environment. Here, we show that dissipation can instead be used to engineer a wide variety of spatio-temporal correlation profiles in an easily tunable manner that is not possible using purely unitary dynamics. We describe how dissipation with any translationally-invariant spatial profile can be realized in cold atoms trapped in an optical cavity. A uniform external field and the choice of spatial profile can be used to design when and how dissipation creates or destroys correlations. We demonstrate this control by preferentially generating entanglement at a desired ‘wavelength’. We thus establish spatially inhomogeneous dissipation as a new route towards engineering the far-from-equilibrium dynamics of quantum information, with potential applications in quantum metrology, state preparation, and transport. | ||
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November 4, 2020 14:00 Virtual Seminar |
Jamir Marino |
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Spatio-temporal control of correlations via inhomogeneous dissipation |
I will discuss the results contained in https://arxiv.org/abs/1909.07381 on how the entanglement spectrum of a region made by adjacent constituents in a one-dimensional quantum system becomes universal after quenches at the critical point or across it. | ||
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July 14, 2020 11:00 Virtual Seminar |
Luca Tagliacozzo |
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Signatures of universality out of equilibrium |
Investigation of strongly interacting quantum field theories (QFTs) remains one of the outstanding challenges of modern physics. Quantum simulation has the potential to be a crucial technique towards solving this problem. By harnessing the power of quantum information processing, quantum simulation can potentially perform tasks deemed intractable by the classical information processing paradigm. In this talk, I will describe analog quantum simulators for strongly interacting QFTs using mesoscopic quantum electronic circuit lattices. The tunable, robust and dispersive Josephson nonlinearity gives rise to the nonlinear interactions in these QFTs… | ||
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June 30, 2020 11:00 Virtual Seminar |
Ananda Roy |
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Quantum Electronic Circuit Simulation of Quantum Field Theories |
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June 23, 2020 11:00 Virtual Seminar |
Jakub Zakrzewski |
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Cold atoms inspired interacting systems: Beyond the ergodic paradigm |
I will review some recent work on the connection between chaos, thermalization, quantum gravity and holography. | ||
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June 16, 2020 11:00 Virtual Seminar |
Jan de Boer |
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Thermalization, chaos and holography |
In this talk I will review the state of the art and the new perspectives in the theoretical and experimental study of analog models of quantum field theories in flat, curved, or time-dependent backgrounds using condensed matter and optical systems. After a brief presentation of the theory and experiments on Hawking emission of phonons from acoustic horizons in quantum fluids of ultracold atoms and of light, I will present recent results (in collaboration with Luca Giacomelli) on superradiance effects in different geometries. In rotating configurations, the instability of multiply charged vortices can be understood in terms of an ergoregion instability at the vortex core. Introduction of synthetic gauge fields in planar geometries extends the range of space-time metrics that can be generated and allows for analytical insight into superradiant scattering processes… | ||
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June 9, 2020 11:00 Virtual Seminar |
Iacopo Carusotto |
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Quantum fields in curved space-times with atomic and optical systems: new directions from synthetic gauge fields and quantum emitters |
I present a novel soliton equation which is related to a particular integrable system of Calogero-Moser-Sutherland (CMS) type. I plan to spend time on background and motivation to our results (starting with Russell’s first observation of solitary waves in 1834). |
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May 26, 2020 11:00 Virtual Seminar |
Edwin Langmann |
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On solitons and Calogero-Moser-Sutherland systems |
Brownian motion is a paradigmatic stochastic process, and well studied both theoretically as well as experimentally. A natural occurance of Brownian motion is found for micron sized particles immersed in simple, Newtonian fluids. The motion of this particle is then a (nearly) perfect random walk, and obeys a linear (Markovian) stochastic equation. This is not true if the particle is suspended in a viscoelastic fluid, which is characterized by long relaxation times and pronounced nonlinear properties. The latter case if thus a good model system for nonlinear stochastic processes, and studying it is challenging, both theoretically as well experimentally. For example, driving the particle drives the viscoelastic fluid out of equilibrium, so that Brownian motion in a non-equilibrium bath is obtained. We will discuss recent theoretical and experimental progress regarding this scenario. | ||
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May 19, 2020 11:00 Virtual Seminar |
Matthias Krüger |
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Brownian Motion in a Non-equilibrium Bath: Theory and Experiment |
Path integrals are a central tool when it comes to describing quantum or thermal fluctuations of particles or fields. Their success dates back to Feynman who showed how to use them within the framework of quantum mechanics. Since then, path integrals have pervaded all areas of physics where fluctuation effects, quantum and/or thermal, are of paramount importance. Their appeal is based on the fact that one converts a problem formulated in terms of operators into one of sampling classical paths with a given weight. Many different definitions are used to define path-integral weight. In statistical mechanics, time-discretization is the standard approach; it implies that, unlike conventional integrals, path integration suffers a serious drawback: in general, one cannot make non-linear changes of variables without committing an error of some sort. In such approach, no path-integral based calculus is possible. We explain which are the mathematical reasons causing this important caveat, and we come up with cures for systems described by one degree of freedom. Our main result is a construction of path integration free of this problem, through a direct time-discretization procedure. We also compare our time-discretized approach to other definitions of path-integral weights that were used in field theories of quantum problems. |
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May 12, 2020 11:00 Virtual Seminar |
Viviene Lecomte |
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Building a path-integral calculus: a covariant discretization approach |
It is often said that perturbation theory is insufficient to understand many physical problems, and that non-perturbative effects are needed. It turns out that making this statement precise requires the mathematical framework of resurgence theory, in which perturbative series are extended to so-called trans-series, and non-perturbative effects can be detected by looking at perturbation theory at large orders. In this talk I will first review the basics of the theory of resurgence, and then present some of its recent applications to condensed matter systems. In particular, I will argue that the superconducting gap can be understood in terms of the Borel singularities of the perturbative series in the normal state, and is similar to the renormalon singularity of quantum chromodynamics. | ||
📅 🕓 🚪 |
April 21, 2020 11:00 Virtual Seminar |
Marcos Mariño |
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Resurgence and non-perturbative physics: applications in condensed matter |
Matrix product states, and operators, are powerful tools for the description of low energy eigenstates and thermal equilibrium states of quantum many-body systems in one spatial dimension. But in out-of-equilibrium scenarios, and for high energy eigenstates of generic systems, the scaling of entanglement with time and system size makes a direct application often impossible. However, MPS and more general TNS techniques can still be used to explore some of the most interesting dynamical properties. We have recently introduced a method in which MPO techniques are combined with Chebyshev polynomial expansions to explore spectral properties of quantum many-body Hamiltonians. In particular, we show how this method can be used to probe thermalization of large spin chains without explicitly simulating their time evolution, as well as to compute full and local densities of states. |
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📅 🕓 🚪 |
April 7, 2020 11:00 Virtual seminar |
Mari Carmen Bañuls |
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Spectral properties and thermalization with matrix product operators |
A central tenant in the classification of phases is that boundary conditions cannot affect the bulk properties of a system. We have uncovered striking, yet puzzling, evidence of a clear violation of this assumption. We use the prototypical example of an XYZ chain with no external field in a ring geometry with an odd number of sites and both ferromagnetic and antiferromagnetic interactions. In such a setting, even at finite sizes, we are able to calculate directly the spontaneous magnetizations that are traditionally used as order parameters to characterize the system’s phases. While when ferromagnetic interactions dominate, we recover the expected behavior, when the system is governed by one antiferromagnetic interaction, the magnetizations decay algebraically to zero with the system size and are not staggered, despite the AFM coupling. We term this behavior ferromagnetic mesoscopic magnetization. With two competing AFM interactions a third, new type of order can emerge, with a magnetization profile that varies in space with an incommensurate pattern… | ||
📅 🕓 🚪 |
March 31, 2020 11:00 Zoom (CMSP – Seminars) |
Fabio Franchini |
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The frustration of being odd: boundary conditions and bulk, local order |
Sine-Gordon model is a paradigmatic integrable quantum field theory featuring strongly correlated dynamics, topological excitations, bound states and strong-weak coupling duality. It is also realised experimentally, allowing us to submit theoretical ideas to reality check. In this talk I overview a number of recent results concerning its non-equilibrium dynamics following a (global) quantum quench, highlighting results as well as outstanding challenges. | ||
📅 🕓 🚪 |
January 28, 2020 11:30 SISSA, Room 128 |
Gábor Takács |
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Quantum Quenches in Sine-Gordon Theory: Progress and Challenges |
(1) When non-interacting Bose-Einstein condensate is confined to a quasi one-dimensional channel it will spread due to dispersion as dictated by the Schrödinger equation. The spreading rate can be affected by changing the interaction between the atoms via the Feshbach resonance. If the interaction is set to just the right value, the attraction between atoms exactly compensates the dispersion. In this case the BEC doesn’t spread and we get a bright matter-wave soliton. (2) Modulating the interaction between the atoms in a Bose-Einstein condensate (BEC) can give raise to diverse phenomena depending on the frequency and amplitude of shaking. When the frequency of modulation is tuned close to collective mode resonance, Faraday waves appear. At low frequencies granulation of BEC is observed, whereas at high frequencies matter-wave jets are emitted. We demonstrate the emission of correlated atom jets from a matter-wave soliton in a quasi-one-dimensional optical trap. All stages of the jet emission are captured in a simple model based on the 1D Gross-Pitaevskii equation (GPE). |
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📅 🕓 🚪 |
January 21, 2020 11:00 SISSA, Room 128 |
Tadej Mežnaršič & Peter Jeglič |
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Cesium matter-wave solitons & Emission of correlated jets from a driven matter-wave soliton |
What would you do if you were a system at criticality confined in a bounded domain? Of course you would forget about details of the interaction, and lattice spacing, flowing to an RG fixed point. Besides attaining this bulk universal behavior you would also try (boundary condition permitting) to forget about the confinement becoming “as uniform as possible”. Implementing this requirement in absolute geometric language, the one used by general relativity, we obtain novel predictions for the structure of one- and two-point correlators. These predictions are tested successfully against numerical experiments yielding a precise estimate of a critical exponent of the Ising model in three dimension. New preliminary results for the three dimensional 3d xy model will also be presented. | ||
📅 🕓 🚪 |
January 16, 2020 15:00 SISSA, Room 128 |
Giacomo Gori |
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Geometry of bounded critical phenomena |
Quantum devices could perform some informational tasks with much better performances than classical systems, with profound implications for cryptography, chemistry, material science, and many areas of physics. However, to reach this goal we need to control large quantum systems, where the many-body dynamics becomes fragile and the system quickly heats up to its thermal state. There are then two key questions: How does a closed quantum system thermalize (thus losing its “quantum power”)? How can we preserve quantum information in the presence of strong interactions? Using a nuclear spin chain as an exemplary experimental system, and the tools of Hamiltonian engineering, I will show how to choreograph the dynamics in order to prevent the system from heating up, even in the presence of strong interactions among spins. In particular, I will show how disorder can quench the scrambling of quantum information, a phenomenon known as localization, and thus prevent thermalization… | ||
📅 🕓 🚪 |
January 9, 2020 11:30 SISSA, Room 128 |
Paola Cappellaro |
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How to avoid “heated” arguments among your spins |
2019 seminars |
Laser cooled trapped ions offer unprecedented control over both internal and external degrees of freedom at the single-particle level. They are considered among the foremost candidates for realizing quantum simulation and computation platforms that can outperform classical computers at specific tasks. In this talk I will show how linear arrays of trapped 171Yb+ ions can be used as a versatile platform for studying out-of-equilibrium strongly correlated many-body quantum systems. In particular I will present our observation of a new type of out-of-equilibrium dynamical phase transition in a spin system with over 50 spins. Moreover, I will show our latest efforts towards scaling up the trapped-ion quantum simulator using a cryo-pumped vacuum chamber where we can trap more than 100 ions indefinitely. The reliable production and lifetime of large linear ion chains enabled us to use up to 40 trapped-ion qubits to observe real-time domain wall confinement in an interacting spin chain and to implement a Quantum Approximate Optimization Algorithm (QAOA) to approximate the ground state energy of a transverse field Ising model. | ||
📅 🕓 🚪 |
December 17, 2019 11:00 ICTP, Stasi Room |
Guido Pagano |
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From Quantum Algorithms to Out-of-Equilibrium Phenomena in Interacting Trapped-Ion Spin Chains |
Motzkin spin chains and their area-weighted deformations are a countinuous family of one-dimensional frustration-free Hamiltonians, whose ground states exhibit a novel quantum phase transition. By tuning a single parameter, they go from a phase obeying an area law to a highly entangled rainbow phase, where the half-chain entropy scales with the volume. Using the representation of these ground states as superpositions of random walks, we introduce tensor networks for these ground states where local and global rules of the walker are baked into bulk tensors, thereby providing an efficient description of the ground states (some of which satisfy a volume law scaling of entanglement entropy). | ||
📅 🕓 🚪 |
November 26, 2019 11:00 SISSA, Room 005 |
Zhao Zhang |
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Motzkin spin chains and their exact holographic tensor network representations |
We explore the intriguing spatial patterns that emerge in a two-dimensional spatially inhomogeneous Katz–Lebowitz–Spohn (KLS) driven lattice gas with attractive nearest-neighbor interactions. The domain is split into two regions with hopping rates governed by different temperatures T > T_{c} and T_{c}, respectively, where T_{c} indicates the critical temperature for phase ordering, and with the temperature boundaries oriented perpendicular to the drive. In the hotter region, the system behaves like the (totally) asymmetric exclusion processes (TASEP), and experiences particle blockage in front of the interface to the critical region. To explain this particle density accumulation near the interface, we have measured the steady-state current in the KLS model at T > T_{c} and found it to decay as 1/T. | ||
📅 🕓 🚪 |
November 21, 2019 10:30 SISSA, Room 005 |
Uwe Tauber |
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Temperature Interfaces in the Katz–Lebowitz–Spohn Driven Lattice Gas |
One of the most interesting current research directions in theoretical high energy physics is studying hardness (complexity) of preparing states or transformation using only simple states and simple operations. This is the essence of conjectured holographic complexity proposals, as well as of ongoing studies of complexity in quantum field theories. In my talk I will discuss complexity in 2-dimensional conformal field theories with a view towards finding a genuine AdS dual of such a notion in quantum field theory. Based on 1904.02713 and an ongoing work with Mario Flory and Volker Schomerus. | ||
📅 🕓 🚪 |
November 12, 2019 11:00 SISSA, Room 005 |
Michal Heller |
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Complexity and conformal field theory |
Characterizing states of matter through the lens of their ergodic properties is a fascinating new direction of research. In the quantum realm, the many-body localization (MBL) was proposed to be the paradigmatic nonergodic phenomenon, which extends the concept of Anderson localization to interacting systems. At the same time, random matrix theory has established a powerful framework for characterizing the onset of quantum chaos and ergodicity (or the absence thereof) in quantum many-body systems. Here we study a paradigmatic class of models that are expected to exhibit MBL, i.e., disordered spin chains with Heisenberg-like interactions. Surprisingly, we observe that exact calculations show no evidence of approaching MBL while increasing disordered strength in the ergodic regime. Moreover, a scaling analysis suggests that quantum chaotic properties survive for any disorder strength in the thermodynamic limit. Our results are based on calculations of the spectral form factor, which provides a powerful measure for the emergence of many-body quantum chaos. | ||
📅 🕓 🚪 |
November 7, 2019 11:30 SISSA, Room 128 |
Lev Vidmar |
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Quantum chaos challenges many-body localization |
Concepts from quantum information theory have become increasingly important in our understanding of entanglement in QFTs. One prominent example of this is the entanglement (or modular) Hamiltonian. Using complex analysis, we determine this operator for the chiral fermion at finite temperature on the circle — which is not fixed by conformal symmetry — and show that it exhibits surprising new features. This simple system illustrates how a modular flow can transition from complete locality to complete non-locality as a function of temperature, thus bridging the gap between previously known limits. We derive the first exact results for the entanglement for the different spin sectors on the torus. | ||
📅 🕓 🚪 |
November 5, 2019 11:00 SISSA, Room 5 |
Ignacio Reyes |
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Entanglement of 2d fermions on the torus |
This is going to be an informal seminar on a current work in progress on the possibility of studying the Ising model using non-local fields on the complex plane. The mapping applies to more general models but we discuss the O(1) for simplicity. We introduce an exact mapping between fields indexed in N with functions on the complex plane – which does not require a continuous limit. The binary character of the Ising model is enforced via an interaction with an auxiliary field whose coupling is the temperature. This is going to be a whiteboard talk, and we focus in particular where to go from here, and the drawbacks and the advantages of using this approach. We show in particular the connection to a U(1) Group Field Theory, a certain parameter scaling limit for the perturbation theory, and end with a discussion on future work: the connection to random matrix theory for the study of glasses in this limit and discuss the constructive approach for this theory. Based on arXiv:1908.08065. | ||
📅 🕓 🚪 |
September 26, 2019 10:00 SISSA, Room 138 |
Francesco Caravelli |
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Continuum field theory for the Ising model |
It has been well known and used extensively that the lowest eigenmodes of the QCD quark Dirac operator are described by random matrix theory. More recently it was shown that the high-temperature cross-over to the quark-gluon plasma state is accompanied by an Anderson-type transition in the quark Dirac spectrum. In the talk I will review some recent results concerning this transition. (The presentation will be quite elementary, in particular no familiarity with QCD will be assumed.) | ||
📅 🕓 🚪 |
September 6, 2019 11:00 SISSA, Room 138 |
Tamás G. Kovács |
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Anderson-type transition of quarks in the quark-gluon plasma |
The understanding of driven-dissipative systems is of fundamental importance to grasp the physics of a large variety of systems, such as photonic quantum simulators and realistic quantum hardware. In this talk, I will review recent developments of this field with particular emphasis on numerical methods. In particular, I will discuss recent applications of neural network tools to simulate the behavior of an open many-body quantum system [1, 2] describing results and open challenges. Next, I will describe how these techniques allow one to study the phase-diagram of paradigmatic strongly-interacting dissipative spin [3, 4] and bosonic [5] systems. Particular attention will be devoted to the stabilization of exotic phases (without an equilibrium counterpart) and to the characterization of criticalities. | ||
📅 🕓 🚪 |
July 18, 2019 11:30 SISSA, Room 132 |
Alberto Biella |
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A neural-network approach to the many-body problem in open quantum system |
We present solutions of the Einstein equations that extend the static Schwarzschild solution in empty space into regions of non-zero energy density ρ and radial pressure P = w/ρ, where w is a constant equation of state parameter. For simplicity we focus mainly on solutions with constant ρ. For w = 0 we find solutions both with and without a singularity at the origin. Possible applications to galaxies are considered, where we find enhanced velocity rotation curves towards the edge of a galaxy. We propose that our explicit non-singular solution with w = −1 describes the interior of a black hole, which is a form of vacuum energy. We verify that its entropy is consistent with the Bekenstein–Hawking entropy, if one assumes the Hawking temperature. We further suggest that this idea can perhaps be applied to the dark energy of the observable universe, if one views the latter as arising from black holes as pockets of vacuum energy. We estimate the average density of such a dark energy to be ρ_{Λ} ≈ 10^{−30} g/cm^{3}. | ||
📅 🕓 🚪 |
June 18, 2019 11:00 SISSA, Room 128 |
Andre Leclair |
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What is inside a Black Hole? |
In 1959 Mark Kac introduced a simple model for the evolution of a gas of hard spheres undergoing elastic collisions. The main simplification consisted in replacing deterministic collisions with random Poisson distributed collisions. It is possible to obtain many interesting results for this simplified dynamics, like estimates on the rate of convergence to equilibrium and validity of the Boltzmann equation. The price paid is that this system has no space structure. I will review some classical results on the Kac model and report on an attempt to reintroduce some form of space structure and non-equilibrium evolution in a way that preserves the mathematical tractability of the system. | ||
📅 🕓 🚪 |
June 13, 2019 11:00 SISSA, Room 4 |
Federico Bonetto |
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The Kac Model and (Non-)Equilibrium Statistical Mechanics |
Extensivity is an essential thermodynamic requirement which is usually broken for long-range correlated and non-exponential growth rate complex systems. The standard approach that deals with this issue is normalization of the system Hamiltonian by a quantity which explicitly depends on the system size (Kac’s prescription). However, as noted by several authors, the prescription does not justify its use from the physical point of view. In this talk we present an alternative approach based on physically consistent generalized thermostatistics which is defined from non-additive entropies and internal energies. The approach is applied for thermostatistical characterization of non-extensive traveling salesman problem. Possible applications to Curie–Weiss model, Sherrington–Kirkpatrick model and Hamiltonian mean field model are also pointed out. | ||
📅 🕓 🚪 |
June 11, 2019 11:00 SISSA, Room 138 |
Velimir Ilić |
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On the extensive generalized thermostatistics for non-extensive complex systems |
Quenched or continuously driven quantum systems can show universal dynamics near non-thermal fixed points, generically in the form of scaling behavior in space and time. Key aspects of the theory of non-thermal fixed points will be briefly summarized, as well as recent experimental results for quenched systems. In a dilute Bose gas, universal scaling dynamics can be due to both linear and non-linear excitations of the system. Considering scaling transport of excitations to larger wave numbers similar to an inverse cascade, the underlying excitations can be either irregular phase excitations or (quasi-)topological defects exhibiting the implications for quantum turbulence. As an example, strongly anomalous scaling of inverse transport in a two-dimensional superfluid due to higher-order vortex annihilation will be discussed both from the theoretical and experimental point of view. | ||
📅 🕓 🚪 |
May 14, 2019 11:00 SISSA, Room 5 |
Thomas Gasenzer |
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Universal Dynamics Near Non-Thermal Fixed Points and Quantum Turbulence |
In my talk I will give a review on the logarithmic terms that appear in the entanglement entropy, their relation to conformal anomaly and the geometry of the entangling surfaces. I will discuss how the presence of boundaries may effect these terms. | ||
📅 🕓 🚪 |
May 7, 2019 11:00 SISSA, Room 128 |
Sergey Solodukhin |
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Logarithmic terms in entanglement entropy: black holes, anomalies and boundaries |
The jamming transition in packings of hard particles is of fundamental interest in the physics of granular materials and glasses. In recent years the physics of jamming has gained momentum in several interdisciplinary contexts, going from Machine Learning to Inference, Ecology and beyond. I will introduce the Simplest Model of Jamming and show how jammed points share peculiar critical properties, highly universal and deeply related glass physics. After discussing the implications of the vicinity to jamming in a glassy phase, I will consider the problem of Information Storage in Machine Learning. Going from the single neuron to multilayer networks, the capacity limit becomes a jamming point. | ||
📅 🕓 🚪 |
April 16, 2019 11:00 ICTP, Stasi Room |
Silvio Franz |
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The Paradigm of Jamming: from Low-Temperature Glasses to Machine Learning and more |
We demonstrate for the first time extremely smooth, coherence-preserving matterwave guides based on time-averaged adiabatic potentials (TAAP). We do so by guiding Bose–Einstein condensates (BEC) over macroscopic distances without affecting their internal coherence: We use a novel magnetic accelerator ring to accelerate BECs to more than 16x their velocity of sound. We transport the BECs in the TAAP over truly macroscopic distances (15 cm) whilst preserving their internal coherence. The BECs can also be released into the waveguide with barriers controllable down to 200 pK giving rise to new regimes of tunnelling and transport through mesoscopic channels. The high angular momentum of more than 40000 ħ per atom and high velocities raises interesting possibilities with respect to the higher Landau levels of quantum Hall states of atoms and open new perspectives in the study of superfluidity. Coherent matterwave guides will result in much longer measurement times (here > 4 s) and much increased sensitivity in highly compact devices. This will raise the spectre of compact, portable guided-atom interferometers for fundamental experiments and applications like gravity mapping or navigation. | ||
📅 🕓 🚪 |
April 15, 2019 11:00 SISSA, Room 128 |
Wolf von Klitzing |
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Hypersonic Transport of Bose-Einstein Condensates in a Neutral-Atom Accelerator Ring |
In this talk, I will discuss a semiclassical numerical method, based on a large-S path integral approach, to study systems whose spin liquid behaviour is underpinned by perturbative ring-exchange Hamiltonians. The method can readily access both thermodynamic and spectral properties. I will focus in particular on quantum spin ice and its photon and vison excitations. After benchmarking the method against existing results on photons, I will show how it can be used to characterise visons and their thermodynamic behaviour. We find that visons form a weak electrolyte — in contrast to spinons in classical spin ice. That is, vison pairs are the dominant population at low temperatures. This is reflected in the behaviour of thermodynamic quantities, such as pinch point motifs in the relevant correlators. Moreover, visons appear to strongly hybridise with the photon background, a phenomenon that likely affects the way these quasiparticles may show up in inelastic response measurements. I will conclude with a brief discussion of the significance of our results and an outlook on further applications of our method. | ||
📅 🕓 🚪 |
April 11, 2019 11:00 ICTP, Stasi Room |
Claudio Castelnovo |
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Seeing Beyond the Light: Vison and Photon Electrodynamics in Quantum Spin Ice |
I will present a class of models for quantum chaos in a spatially extended many-body system. It consists of a chain of sites with nearest-neighbour coupling under Floquet time evolution. Quantum states at each site span a q-dimensional Hilbert space and the time evolution is specified as a random circuit, whose local gates are random in space but periodic in time (Floquet). I will discuss a diagrammatic formalism useful to average over realisations of the random circuit.
This approach leads to exact expressions in the large-q limit and sheds light on the universality of random matrices in many-body quantum spectra and the ubiquitous entanglement growth in out-of-equilibrium dynamics. I will also discuss universal behaviour which goes beyond random matrix theory and the manifestation of ergodicity breaking which can emerge at finite q. |
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📅 🕓 🚪 |
March 19, 2019 11:00 ICTP, Stasi Room |
Andrea De Luca |
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Solvable minimal models for many-body quantum chaos |
The simplest model of granular material is a “fluid” made of inelastic hard spheres. For such a system—in the dilute limit—the classical program of kinetic theory Boltzmann equation, Chapman–Enskog-based hydrodynamics has been developed by physicists and mathematicians in the last decades. In this seminar, after recalling a few key results of such a theoretical activity, I will focus on a series of experiments made in my laboratory in the last 5 years. They concern the statistical properties of a massive probe immersed in a steady state granular fluid. The fluid is obtained by vibro-fluidization of a large number of solid spheres of different materials, while the probe is a rigid rotator whose angular displacement and angular velocity are the key observables. In the dilute limit one conjectures a Markovian approximation for the rotator’s dynamics which explains many aspects of the experiment, including a qualitative understanding of “motor effects” in the presence of rotator’s geometrical asymmetries. Further noticeable facts appear when the granular fluid is no more dilute, mainly anomalous diffusion and non-monotonous viscosity. |
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📅 🕓 🚪 |
March 12, 2019 11:00 SISSA, Room 128 |
Andrea Puglisi |
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Granular Brownian Motion |
We study inhomogeneous quenches in integrable models. The Non-Equilibrium Steady State emerging in such systems has been recently conjectured to be described by a Generalised Hydrodynamic theory. We develop a mathematically rigorous method to calculate the asymptotics of observables at large times and distances and show how certain predictions of this conjecture can be derived from analyticity properties of the Slavnov formula for Bethe state overlaps. | ||
📅 🕓 🚪 |
March 5, 2019 11:00 SISSA, Room 128 |
Spyros Sotiriadis |
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Quantum Transport after Inhomogeneous Quenches |
The determination of four-point correlation functions of two-dimensional lattice models is of fundamental importance in statistical physics. In the limit of an infinite lattice, this question can be formulated in terms of conformal field theory (CFT). For the so-called minimal models the problem was solved more than 30 years ago, by using that the existence of singular states implies that the correlation functions must satisfy certain differential equations. This settles the issue for models defined in terms of local degrees of freedom, such as the Ising and 3-state Potts models. However, for geometrical observables in the Fortuin–Kasteleyn cluster formulation of the Q-state Potts model, for generic values of Q, there is in general no locality and no singular states, and so the question remains open. As a warm-up to solving this problem, we discuss which states propagate in the s-channel of such correlation functions, when the four points are brought together two by two. To this end we combine CFT methods with algebraic and numerical approaches to the lattice model. | ||
📅 🕓 🚪 |
February 19, 2019 11:00 SISSA, Room 128 |
Jesper Jacobsen |
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Four-point functions in the Fortuin–Kasteleyn cluster model |
I will present recent work on ferromagnetic quantum Hall states that form on (111) surfaces of elemental Bismuth in high magnetic fields. This unusual states of matter combine the topological features of quantum Hall states with orientational symmetry breaking characteristic of nematic order. Recent scanning tunneling microscopy measurements have directly visualized the spontaneous formation of boundary modes between distinct nematic domains and investigated their electronic structure. I will demonstrate that these boundary modes belong to a new class of `symmetry-protected’ Luttinger liquid that arise from the interplay of symmetry-breaking with quantum Hall physics, and that they provide a concrete realization of ‘anomaly inflow’. The analysis reveals strikingly different behavior of domain wall transport at quantum Hall filling factor ν = 1.2, in striking agreement with the STM results. I will explore implications of these ideas for the global phase diagram of quantum Hall valley nematics. | ||
📅 🕓 🚪 |
January 29, 2019 11:00 ICTP, Stasi Room |
Siddharth Parameswaran |
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Topology, symmetry, and anomalies: investigating domain wall physics in quantum Hall nematic states |
One of the paradigms of quantum mechanics is the statistical nature of measurements: the result of measurements is indeed described by a probability distribution function (PDF), and measuring the same observable in identical systems will give different outcomes in accordance with this distribution. The PDF carries very detailed information about the system, going much beyond the simple average. Here I exploit the Matrix Product Operator (MPO) representation of the Generating Functions to efficiently perform local measurements in one-dimensional spin systems using Tensor Network Methods, both in and out-of-equilibrium. Finally, inspired by such formulation, I show some preliminary results connecting MPS with Neural Network Quantum States. | ||
📅 🕓 🚪 |
January 24, 2019 14:00 SISSA, Room 128 |
Mario Collura |
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Tensor Network Methods for Probability Distribution Functions and beyond… |
Several atomic, molecular, and optical systems, as well as certain condensed matter models, exhibit long-range interactions that decay with distance r as a power law 1/r^{α}. In this talk, we will present recent results for the localization properties of correlation functions of these long-range quantum models in the presence of disorder. The latter is usually associated with exponential localization of wave functions and correlations. We demonstrate that in most situations in 1D power-law interactions imply algebraic decay of correlations. We will discuss the generality of these results and their application to experiments in atomic and molecular physics. | ||
📅 🕓 🚪 |
January 22, 2019 11:00 SISSA, Room 128 |
Guido Pupillo |
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Algebraic localization of disordered long-range quantum models |
Quantum optimal control allows one to find the optimal strategy to drive a quantum system into a target state. We review an efficient algorithm to optimally control many-body quantum dynamics and apply it to quantum annealing, going beyond the adiabatic strategy. We present an information theoretical analysis of quantum optimal control processes and its implications. We review some recent advancements we have obtained in tensor network algorithms that enable such investigations and that can be exploited to support the development of quantum technologies via classical numerical simulations: novel approaches to study abelian and non-abelian lattice gauge theories, open many-body quantum systems and systems with long-range interactions or periodic boundary conditions. Finally, we report some theoretical and experimental applications of these approaches to relevant scenarios, such as Rydberg atoms in optical lattices and the gauge theory resulting from the mapping of classical hard problems to short-range quantum Hamiltonians. |
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📅 🕓 🚪 |
January 15, 2019 11:00 ICTP, Stasi Room |
Simone Montangero |
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Optimal control, lattice gauge theories, and quantum annealing |
2018 seminars |
In this talk, I briefly review recent experimental advances in the generation of topological band structures in the non-interacting regime using Floquet engineering and present first studies of interacting atoms in driven 1D lattices. In particular, I will present experimental results obtained with bosonic atoms in driven 1D lattices that directly reveal the existence of parametric instabilities that lead to a depletion of the condensate. Our results point out ways to overcome these limitations in future experiments. In the last part of my talk I will present recent results, where we have used a combination of periodic modulation and strong Hubbard interactions to realize a minimal building block of Z2 lattice gauge theories. We engineer a minimal coupling between matter and gauge fields using two different internal states of bosonic Rb atoms. The obtained lattice model displays local Z2 gauge symmetry, which we study experimentally in a double-well potential – the building block of extended lattice models. |
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📅 🕓 🚪 |
December 18, 2018 11:00 ICTP, Stasi Room |
Monika Aidelsburger |
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Ultracold atoms in periodically-driven optical lattices |
The infrared fixed point of graphene under the renormalization group flow is a relatively under studied yet important example of a boundary conformal field theory with a number of remarkable properties. It has a close relationship with three dimensional QED. It maps to itself under electric-magnetic duality. Moreover, it along with its supersymmetric cousins all possess an exactly marginal coupling — the charge of the electron — which allows for straightforward perturbative calculations in the weak coupling limit. I will review past work on this model and also discuss my own contributions, which focus on understanding the boundary contributions to the anomalous trace of the stress tensor and their role in helping to understand the structure of boundary conformal field theory. | ||
📅 🕓 🚪 |
December 11, 2018 11:00 SISSA, Room 128 |
Christopher Herzog |
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Graphene and Boundary Conformal Field Theory |
Critical lattice models with a non-hermitian Hamiltonian are described by non-unitary CFTs. There are many physical applications such as open quantum systems, geometrical problems or electronic disordered systems. We know that in many cases we can define the notion of effective central charge.I will show why this quantity is important in two problems: the scaling of the entanglement entropy and the identification of universality classes in truncated models. I will illustrate the discussion with examples such as the XXZ model, supersymmetric spin chains, loop models and truncations of the Brownian motion. | ||
📅 🕓 🚪 |
November 27, 2018 11:00 SISSA, Room 128 |
Romain Couvreur |
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Role of the effective central charge in non-unitary conformal field theories |
We study a non-unitary spin chain with orthosymplectic symmetry that generalizes the O(N) model to any positive or negative integer N. The lack of unitarity allows a stable massless Goldstone phase to appear, otherwise forbidden by the Mermin–Wagner theorem, that is described by a supersphere sigma model. On the 2D lattice it is represented as a dense loop model with loop weight N in which crossings are allowed. Unlike the usual O(N) loop model, the presence of crossings makes the model flow to a different regime where correlations involve logarithms. We compute these logarithmic critical exponents with field theory and the Bethe ansatz. | ||
📅 🕓 🚪 |
November 20, 2018 11:00 SISSA, Room 128 |
Etienne Granet |
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A study of a non-unitary statistical model: super spin chains and intersecting loops |
We demonstrate the existence of a new quantum phase of matter that arises in antiferromagnetic spin chains with a weak frustration—just one bond in a large chain. This is the case, for instance, of systems with an odd number of spins with periodic boundary conditions. Such new phase is extended, gapless, but not relativistic: the low-energy excitations have a quadratic (Galilean) spectrum. Locally, the correlation functions on the ground state do not show significant deviations compared to the non-frustrated case, but correlators involving a number of sites (or distances) scaling like the system size display new behaviors. In particular, the von Neumann entanglement entropy is found to follow new rules, for which neither area law applies, nor one has a divergence of the entropy with the system size. Such very long-range correlations are novel and of potential technological interest. We display such new phase in a few prototypical chains using numerical simulations and we study analytically the paradigmatic example of the Ising chain. Through these examples we argue that this phase emerges generally in (weakly) frustrated systems with discrete symmetries. | ||
📅 🕓 🚪 |
November 6, 2018 11:00 SISSA, Room 4 |
Fabio Franchini |
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The Frustration in being Odd: area law violation in local systems |
I will discuss walking behavior in gauge theories and weakly first order phase transition in statistical models. Despite being phenomena appearing in very different physical systems, they both show a region of approximate scale invariance. They can be understood as a theory passing between two fixed points living at complex couplings, which we call complex CFTs. By using conformal perturbation theory, knowing the conformal data of the complex CFTs allows us to make predictions on the observables of the walking theory. As an example, I will discuss the two dimensional Q-state Potts model with Q>4. | ||
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October 16, 2018 11:00 SISSA, Room 4 |
Bernardo Zan |
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Walking behavior, weakly first order phase transitions and complex CFTs |
In this talk, I will discuss an exact mapping between many-body quantum spin systems and classical stochastic processes. This approach can handle integrable and non-integrable systems, including those in higher dimensions, in a unified framework, and can be applied both in and out of equilibrium. Focusing on quantum quenches, I will discuss dynamical quantum phase transitions in the Loschmidt amplitude, showing that these correspond to enhanced fluctuations and other features in the classical stochastic coordinates. | ||
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October 9, 2018 11:00 SISSA, Room 138 |
Stefano De Nicola |
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A Stochastic Approach to Quantum Spin Systems |
We revisit the calculation of multi-interval modular Hamiltonians for free fermions using a Euclidean path integral approach. We show how the multi-interval modular flow is obtained by gluing together the single interval modular flows. Our methods are based on a derivation of the non-local field theory describing the reduced density matrix, and makes manifest its non-local conformal symmetry and U(1) symmetry. We will show how the non local conformal symmetry provides a simple calculation of the entanglement entropy. Time-permitting, we will connect multi-interval modular flows to the frame work of extended quantum field theory. | ||
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September 18, 2018 11:00 SISSA, Room 128 |
Gabriel Wong |
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Gluing together modular flows with free fermions |
Consider a quantum chain in its ground state and then take a subdomain of this system with natural truncated Hamiltonian. Since the total Hamiltonian does not commute with the truncated Hamiltonian the subsystem can be in one of its eigenenergies with different probabilities. Since the global energy eigenstates are locally close to diagonal in the local energy eigenbasis we argue that the Shannon (Rényi) entropy of these probabilities follows an area-law for the gapped systems. When the system is at the critical point the Shannon (Rényi) entropy follows a logarithmic behaviour with a universal coefficient. Our results show that the Shannon (Rényi) entropy of the subsystem energies closely mimics the behaviour of the entanglement entropy in quantum chains. We support the arguments by detailed numerical calculations performed on the transverse field XY chain. | ||
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July 31, 2018 11:10 ICTP, Stasi Room |
Mohammad Ali Rajabpour |
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Area-law and universality in the statistics of the subsystem energy |
Integrated Information Theory (IIT) has emerged as one of the leading research lines in computational neuroscience to provide a mechanistic and mathematically well-defined description of the neural correlates of consciousness. Integrated Information quantifies how much the integrated cause/effect structure of the global neural network fails to be accounted for by any partitioned version of it. The holistic IIT approach is in principle applicable to any information-processing dynamical network regardless of its interpretation in the context of consciousness. In this talk I will describe the first steps towards a possible formulation of a general and consistent version of IIT for interacting networks of quantum systems irrespective of potential applications to consciousness. A variety of different phases, from the dis-integrated to the holistic one can be identified and their cross-overs studied. | ||
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July 3, 2018 11:00 SISSA, Room 128 |
Paolo Zanardi |
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Quantum Integrated Information Theory |
Models for active matter have brought a new type of experiments in statistical physics where the source of nonequilibrium lies within the particles themselves or on their surface. In this talk, I will take the viewpoint of molecular simulations to study matching experiments on chemically-powered anomotors: self-propulsion by symmetry-breaking, chemotaxis, sedimentation and anisotropic nanomotors. I will comment on the design of consistent microscopic models with respect to energy conservation, to chemical kinetic, and to thermal fluctuations. As a perspective, I will discuss enzyme nanomotors. On the one hand, they consist in elaborate catalytic devices with interesting thermodynamic properties and on the other hand they might inspire or serve as molecular scale machine for nano- and bio-technology in the coming years. | ||
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June 19, 2018 11:00 SISSA, Room 128 |
Pierre de Buyl |
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Nanomotors: symmetry, chemotaxis, sedimentation and anisotropy |
The partial transpose of density matrices in many-body systems has been known as a good candidate to diagnose quantum entanglement of mixed states. In particular, it can be used to define the (logarithmic) entanglement negativity for bosonic systems. In this talk, I introduce partial time-reversal transformation as an analog of partial transpose for fermions. This definition naturally arises from the spacetime picture of partially transposed density matrices in which partial transpose is equivalent to reversing the arrow of time for one subsystem relative to the other subsystem. I show the success of this definition in capturing the entanglement of fermionic symmetry-protected topological phases as well as conformal field theories in (1+1) dimensions. | ||
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June 18, 2018 11:00 SISSA, Room 128 |
Hassan Shapourian |
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Partial time-reversal transformation and entanglement negativity in fermionic systems |
The Schur process is in some sense a discrete analogue of a random matrix. Their edge behavior are known to be in the same universality class, described by the Airy kernel and the Tracy–Widom distribution. In this talk we consider two variants of the Schur process: the periodic case introduced by Borodin, and the “free boundary” case recently introduced by us. We are able to compute their correlation functions in a unified manner using the machinery of free fermions. We then investigate the edge asymptotic behavior and show it corresponds to two nontrivial deformations of the Airy kernel and of the Tracy–Widom distribution. Based on joint work with Dan Betea, Peter Nejjar and Mirjana Vuletić. | ||
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May 29, 2018 11:00 SISSA, Room 128 |
Jeremie Bouttier |
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Edge behavior of the periodic and the free boundary Schur processes |
Every physicist has a pretty clear idea of how to define equilibrium phases of matter (e.g. using free energy considerations), whether disordered or ordered (and if ordered, a variety of situations can be encountered). By contrast, dynamics-wise, no generic and clear-cut definition a dynamical phase (disordered, intermittent, uniform, ergodicity-breaking, pattern-forming, etc) can be found. Instead, one works on a system-to-system basis. I will illustrate, on the simple example of a classical system of mutually excluding particles diffusing on a line, how a robust definition of what a dynamical phase is can be achieved. As I will go along, we will see that there may even exist transitions between dynamical phases. On a formal level, these dynamical transitions have everything in common with the quantum phase transitions that appear in hard-condensed matter. I will show that, in turn, approaching quantum problems with a classical eye, can, even with the simple example I’ll discuss, lead to unexpected progress on the quantum side. |
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May 8, 2018 11:00 SISSA, Room 128 |
Frédéric van Wijland |
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Dynamical phase transitions |
This talk addresses the low energy physics of the Sachdev–Ye–Kitaev model, a paradigm of strongly interacting (Majorana) quantum matter. A salient feature of this system is its exceptionally high degree of symmetry under reparameterizations of physical time. At low energies this symmetry is spontaneously broken and the ensuing infinite dimensional Goldstone mode manifold takes strong influence on all physical observables. We will discuss the effects of these fluctuations on the example of the so-called out of time ordered correlation functions, diagnostic tools to describe both manifestations of quantum chaos in the system and its conjectured duality to an AdS_{2} gravitational bulk. While previous work predicts exponential decay of these correlations in time our main finding is that at large time scales non-perturbative Goldstone mode fluctuations generate a crossover to power law behavior. This phenomenon must have ramifications in the physics of the holographic bulk which, however, we do not understand at present. | ||
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April 24, 2018 11:00 ICTP, Stasi Room |
Alex Altland |
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Large Conformal Goldstone Mode Fluctuations in the SYK Model |
The grand canonical ensemble lies at the core of quantum and classical statistical mechanics. A small system thermalizes to this ensemble while exchanging heat and particles with a bath. A quantum system may exchange quantities represented by operators that fail to commute. Whether such a system thermalizes and what form the thermal state has are questions about truly quantum thermodynamics. Here we investigate this thermal state from three perspectives. First, we introduce an approximate microcanonical ensemble. If this ensemble characterizes the system-and-bath composite, tracing out the bath yields the system’s thermal state. This state is expected to be the equilibrium point, we argue, of typical dynamics. Finally, we define a resource-theory model for thermodynamic exchanges of noncommuting observables. Complete passivity — the inability to extract work from equilibrium states — implies the thermal state’s form, too. Our work opens new avenues into equilibrium in the presence of quantum noncommutation. [Based on 1512.01189 with N. Yunger Halpern, P. Faist and J. Oppenheim.] |
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April 17, 2018 11:00 ICTP, Stasi Room |
Andreas Winter |
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Microcanonical and resource-theoretic derivations of the grand canonical thermal state of a system with non-commuting charges |
In this talk I will describe our work on the simulation of the Schwinger model (i.e. d=1+1 QED) with matrix product states (MPS). I will discuss some systematic aspects of our approach like the truncation of the local infinite bosonic gauge field Hilbert space, or the incorporation of local gauge invariance into the MPS ansatz. Furthermore, I will go through some of our results: the simulation of the particle excitations (“mesons” of confined electron/positron pairs), of string breaking for heavy probe charges and last but not least of the real-time evolution that occurs from a background electric field quench (i.e. the full quantum Schwinger effect). | ||
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March 27, 2018 11:00 ICTP, Stasi Room |
Karel Van Acoleyen |
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Matrix product states for relativistic quantum gauge field theories |
I will first start with a general introduction on theoretical ecology, stressing the reasons that make connections with statistical physics interesting and timely. I will then focus on Lotka–Volterra equations, which provide a general model to study large assemblies of strongly interacting degrees of freedom in many different fields: biology, economy and in particular ecology. I will present our analysis of Lotka–Volterra equations as model of ecosystems formed by a large number of species and show the different phases that emerge. Two of them are particularly interesting: when interactions are symmetric we find a regime characterised by an exponential number of multiple equilibria, all poised at the edge of stability for a large number of species. For non symmetric interactions, this phase is replaced by a chaotic one. I will then conclude discussing relationships with experiments and general consequences of our works. |
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March 21, 2018 11:00 SISSA, Room 005 |
Giulio Biroli |
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Emergent phenomena in large interacting ecosystems |
I will discuss several recent results, both numerical and analytical, regarding disordered models in external field, focusing mainly on random field ferromagnetic models and spin glasses in a field. I will mainly treat models with Ising variables, but also some new results on XY models will be presented. Exact analytical results are derived for models defined on random graphs under the Bethe approximation, while numerical results are obtained via large scale Monte Carlo simulations for finite dimensional models and via improved message passing algorithms for models on random graphs. | ||
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March 13, 2018 11:00 SISSA, Room 128 |
Federico Ricci-Tersenghi |
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On the complex behavior of disordered models in a field |
In August 1859 the young and still little known Bernhard Riemann presented a paper to the Berlin Academic titled “On the number of primes less than a given quantity”. In the middle of that paper, Riemann made a guess — remark or conjecture — on the zeros of analytic function which controls the growth of the primes. Mathematics has never been the same since. The seminar presents the captivating story behind this problem and discusses how the original conjecture can be extended to all Dirichlet functions, giving rise to the Generalised Riemann Hypothesis for the non-trivial zeros of all these functions. We show that the solution of the Generalised Riemann Hypothesis can be obtained employing ideas and methods which come statistical physics, i.e. from the stochastic world of random walks and alike. |
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February 28, 2018 11:00 SISSA, Room 128 |
Giuseppe Mussardo |
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The Riemann conjecture |
We study the XXZ spin chain in the presence of a slowly varying magnetic field gradient. First, it is shown that a local density approximation perfectly captures the ground-state magnetization profile. Furthermore, we demonstrate how the recently introduced technique of curved-spacetime CFT yields a very good approximation of the entanglement profile. Finally, the front dynamics is also studied after the gradient field has been switched off. | ||
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February 27, 2018 11:00 SISSA, Room 005 |
Viktor Eisler |
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Entanglement in the XXZ chain with a gradient |
(Boltzmann lecture) I will address one of the fundamental questions in statistical physics: how to conciliate the laws of quantum mechanics for a macroscopic system — which predict a memory of the initial state of the system — with the familiar irreversible phenomena that bring any extended system to a thermal equilibrium, where all memory of the initial state is lost. I will present a series of new results on cold atom quantum systems made of mixtures of fermions, which lead to a physical phenomenon known as Many Body Localization Transition. Moreover, I will discuss the possibility to realize quantum systems with negative temperature in the laboratory. | ||
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February 20, 2018 11:00 SISSA, Room 128 |
Immanuel Bloch |
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Using Ultracold Quantum Gases to Probe New and Old Frontiers of Statistical Physics |
The Tan’s contact is an ubiquitous quantity in systems with zero-range interactions: it corresponds for example to the average interaction energy, to the weight of the tails of the momentum distribution function at large momenta, to the inelastic two-body loss rate, just to cite a few. We focus on strongly interacting one-dimensional bosons at finite temperature under harmonic confinement. As it is associated to short-distance correlations, the calculation of the Tan’s contact cannot be obtained within the Luttinger-liquid formalism. We derive the Tan’s contact by employing an exact solution at infinite interactions, as well as a local-density approximation on the Bethe Ansatz solution for the homogeneous system and numerical ab initio calculations for finite interactions. In the limit of infinite interactions, we demonstrate its universal properties, associated to the scale invariance of the model. We then obtain the full scaling function for arbitrary interactions. | ||
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February 19, 2018 11:00 ICTP, Stasi Room |
Anna Minguzzi |
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Tan’s contact for a strongly interacting one-dimensional Bose gas in harmonic confinement: universal properties and scaling functions |
In this talk I will discuss the motion of a tracer particle driven by an external constant force through a quiescent lattice gas. Due to the interaction between the tracer and the bath particles, here modelled as an exclusion process, the driven tracer reaches a steady-state when the external force and the friction exerted by the bath balance each other. The steady-state is characterised by a non equilibrium broad inhomogeneity of the bath density surrounding the driven tracer yielding a rich variety of behaviours. I show that depending on the effective dimension of the lattice, the driven tracer exhibits from sub-diffusive to strong super-diffusive transport in the limit of high of bath particles. Moreover, when more than one driven tracers exist, the external and friction forces mediate an anisotropic attractive interacting force between the tracers, leading to the formation of clusters. I will show through numerical results that such scenario extends into continuous-space and continuous-time dynamics. | ||
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February 13, 2018 11:00 SISSA, Room 128 |
Carlos Mejía Monasterio |
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Driven tracer in quiescent baths: anomalous diffusion and induced-interaction |
Irreversibility, which is usually quantified by the entropy production, is one of the most fundamental concepts in thermodynamics, with deep scientific and technological consequences. It is also an emergent concept, that stems from the complex interactions between a system and its environment. However, as will be discussed in this talk, the standard theory of entropy production breaks down in the quantum case, in particular in the limit of zero temperature. Motivated by this, I will present recent results which overcome these difficulties using the idea of phase space entropy measures for bosonic systems. As I will show, our theory not only overcomes the zero temperature limitations but also allows one to extend the results to deal with non-equilibrium reservoirs. As an application, we will consider squeezed thermal baths, which are instance of a grand-canonical Generalized Gibbs Ensemble and therefore allow us to construct an Onsager transport theory, akin to the theory of thermoelectricity. Finally, I will also discuss how entropy production emerges from the perspective of the environment and the system environment correlations. | ||
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February 6, 2018 11:00 SISSA, Room 128 |
Gabriel Landi |
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Measures of irreversibility in quantum phase space |
We present a new method to compute Rényi entropies in one-dimensional critical systems using the mapping of the Nth Rényi entropy to a correlation function involving twist fields in a ℤ_{N} cyclic orbifold. When the CFT describing the universality class of the critical system is rational, so is the corresponding cyclic orbifold. It follows that the twist fields are degenerate: they have null vectors. From these null vectors a Fuchsian differential equation is derived, although this step can be rather involved since the null-vector conditions generically involve fractional modes of the orbifold algebra. The last step is to solve this differential equation and build a monodromy invariant correlation function, which is done using standard bootstrap methods. This method is applicable in a variety of situations where no other method is available, for instance when the subsystem A is not connected (e.g. two-intervals EE). | ||
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January 30, 2018 11:00 SISSA, Room 128 |
Benoit Estienne |
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Entanglement entropies of 1d critical systems, orbifold and null-vectors |
By the eigenstate thermalization hypothesis (ETH), a highly excited energy eigenstate behaves like a thermal state. It is related to the black hole information paradox by the AdS/CFT correspondence. I will talk about ETH in two-dimensional large central charge CFT and compare the excited state of a primary operator with the thermal state. To define ETH precisely, one needs to know how similar, or equivalently dissimilar, the excited state and thermal state are. I will talk about short interval expansions of the entanglement entropy, relative entropy, Jensen–Shannon divergence. For the canonical ensemble, the excited state and thermal state are the same at the leading order of large central charge and are different at the next-to-leading order. I will also discuss briefly ETH for generalized Gibbs ensemble, and ETH for the descendant excited states. | ||
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January 25, 2018 14:00 SISSA, Room 138 |
Jia-Ju Zhang |
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Eigenstate thermalization hypothesis in two-dimensional large central charge CFT |
In this talk I will motivate the interest for studying SU(N) quantum magnetism, and present three recent results on: i) a microscopic model exhibiting SU(N) chiral spin liquids and their characterization, ii) the phase diagram of SU(N) two-leg spin ladders and iii) finite temperature “phase diagrams” of SU(N) Heisenberg models on two-dimensional lattices. |
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January 23, 2018 11:00 ICTP, Stasi Room |
Andreas Läuchli |
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SU(N) Quantum Magnetism in 1D and 2D |
Recent experiments on large chains of Rydberg atoms [H. Bernien et al., arXiv:1707.04344] have demonstrated the possibility of realizing 1D systems with locally constrained Hilbert spaces, along with some surprising signatures of non-ergodic dynamics, such as persistent oscillations following a quench from the Neel product state. I will argue that this phenomenon is a manifestation of a “quantum many-body scar”, i.e., a concentration of extensively many eigenstates of the system around special many-body states. The special states are analogs of unstable classical periodic orbits in the single-particle quantum scars. I will present a model based on a single particle hopping on the Hilbert space graph, which quantitatively captures the scarred wave functions up to large systems of 32 atoms. These results suggest that scarred many-body bands give rise to a new universality class of quantum dynamics, which opens up opportunities for creating and manipulating novel states with long-lived coherence in systems that are now amenable to experimental study. | ||
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January 16, 2018 11:00 ICTP, Stasi Room |
Zlatko Papic |
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Quantum Many-body Scars and Non-ergodic Dynamics in the Fibonacci Chain |
Strongly correlated quantum systems exhibit a wide range of phases with unconventional behavior. These phases are characterized by non-trivial global entanglement patterns and cannot be described within the Landau paradigm due to their lack of local order parameters. In my talk, I will discuss how quantum information theory allows us to describe such systems in a way which reconciles their global entanglement with a local description, based on the framework of tensor networks. I will show how tensor networks allow to capture both the structure of the physical interactions as well as global topological entanglement within a unified local description, and how this allows us to build a comprehensive framework to study topologically ordered systems and their excitations. I will then discuss applications of this framework: First, I will show how it allows to characterize the precise nature of topological spin liquids; and second, I will discuss how it can be used to explain topological phase transitions driven by anyon condensation through phases in their entanglement, allowing us to devise measurable order parameters for anyon condensation and thus to study topological phase transitions at a microscopic level. | ||
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January 9, 2018 11:00 ICTP, Stasi Room |
Norbert Schuch |
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Topological Order and Tensor Networks: A Local Perspective on Global Entanglement |
2017 seminars |
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December 19, 2017 11:00 SISSA, Room 128 |
Marco Baiesi |
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Entanglement in protein native states |
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December 12, 2017 11:00 ICTP, Stasi Room |
Matteo Polettini |
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Effective thermodynamics for a marginal observer |
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December 5, 2017 11:00 ICTP, Stasi Room |
Achilleas Lazarides |
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Floquet Systems-Ensembles and Order Under Periodic Driving |
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November 28, 2017 12:00 ICTP, Stasi Room |
Alessandro Vezzani |
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Single big jump and probability condensation in correlated random walks: the case of Lévy Lorentz gas |
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November 14, 2016 11:00 SISSA, Room 005 |
Ingo Peschel |
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The Entanglement Hamiltonian of a Free-Fermion Chain | ||
Watch online |
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November 7, 2017 11:00 SISSA, Room 128 |
Maurizio Fagotti |
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Beyond (first-order) generalized hydrodynamics: why? and how!? |
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October 17, 2017 11:00 SISSA, Room 128 |
Sascha Wald |
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Thermalisation and Relaxation of Quantum Systems | ||
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📅 🕓 🚪 |
October 11, 2017 11:00 SISSA, Room 005 |
Juan R. Gomez-Solano |
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Self-propelled colloidal particles in viscoelastic fluids | ||
Watch online |
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October 4, 2017 11:00 SISSA, Room 128 |
Enrique Rico Ortega |
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Exploring SO(3) “Nuclear Physics” with Ultra-cold Gases |
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September 26, 2017 11:00 SISSA, Big Meeting Room |
Alessandro Codello |
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Functional perturbative RG and CFT data in the ε-expansion | ||
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📅 🕓 🚪 |
September 18, 2017 11:00 ICTP, Stasi Room |
Markus Müller |
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Creating Cool Quantum Matter by Non-linear Driving |
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September 4, 2017 11:00 SISSA, Room 128 |
Fabian H.L. Essler |
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Quantum Master Equations and Integrability |
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June 6, 2017 11:00 ICTP, Stasi Room |
Pranjal Bordia |
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Many-Body Localization Through the Lens of Ultracold Quantum Gases | ||
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📅 🕓 🚪 |
May 23, 2017 11:00 SISSA, Room 138 |
Masud Haque |
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Non-equilibrium dynamics in isolated quantum systems | ||
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May 4, 2017 11:00 SISSA, Room 138 |
P.K. Mohanty |
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Zeroth law in non-equilibrium — a hot needle in water | ||
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📅 🕓 🚪 |
May 2, 2017 11:00 ICTP, Stasi Room |
G. Biroli |
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Non-Linear Responses, Soft Modes and the True Nature of Glasses | ||
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📅 🕓 🚪 |
April 27, 2017 15:00 SISSA, Room 138 |
S. Sinha |
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Recent developments in Quantum Chaos | ||
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📅 🕓 🚪 |
April 20, 2017 11:00 SISSA, Room 138 |
A. Bernamonti |
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Heavy–Heavy–Light–Light correlators in Liouville theory | ||
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📅 🕓 🚪 |
April 18, 2017 11:00 ICTP, Stasi Room |
R. Moessner |
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Thermodynamics and Order Beyond Equilibrium — The Physics of Periodically Driven Quantum Systems | ||
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📅 🕓 🚪 |
April 12, 2017 14:00 SISSA, Room 138 |
F. Galli |
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Entanglement scrambling in 2d CFT | ||
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📅 🕓 🚪 |
April 11, 2017 11:00 SISSA, Room 128 |
G. Santoro |
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Floquet Topological Insulators? A few warnings | ||
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📅 🕓 🚪 |
March 28, 2017 11:00 SISSA, Room 128 |
F.S. Cataliotti |
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Quantum Control on an Atom Chip | ||
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📅 🕓 🚪 |
March 23, 2017 11:00 SISSA, Room 138 |
N. Pranjal |
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Virasoro coadjoint orbits of SYK/tensor-models & Emergent 2-D Quantum Gravity | ||
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📅 🕓 🚪 |
March 21, 2017 11:00 ICTP, Stasi Room |
A. Rosso |
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Liouville Field Theory and Log-correlated Random Energy Models | ||
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📅 🕓 🚪 |
March 16, 2017 11:00 SISSA, Room 138 |
A. de Quieroz |
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Dualities and Symmetries in the Entanglement Entropy of Fermionic Chains |
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March 14, 2017 11:00 SISSA, Room 128 |
T. Roscilde |
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Quantum correlations: equilibrium and non-equilibrium aspects | ||
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📅 🕓 🚪 |
February 28, 2017 11:00 SISSA, Room 128 |
I. Lesanovsky |
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Exploring far-from-equilibrium physics of dissipative spin systems with highly excited atoms |
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February 21, 2017 12:00 ICTP, Stasi Room |
S. Ciliberto |
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A Protocol for Reaching Equilibrium Arbitrarily Fast |
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February 2, 2017 14:00 SISSA, Room 128 |
J. Viti |
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📅 🕓 🚪 |
January 26, 2017 11:00 SISSA, Room 128 |
G. Parisi |
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The physics of jamming: a journey from marble pebbles toward scaling invariant field theory | ||
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📅 🕓 🚪 |
January 17, 2017 11:00 SISSA, Room 128 |
S. Simon |
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Big Surprises from Small Quantum Hall Droplets | ||
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January 11, 2017 16:30 SISSA, Room 128 |
N. Defenu |
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Watch online |
2016 seminars |
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November 22, 2016 11:30 SISSA, Room 128 |
M. Serone |
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The Effective Bootstrap |
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November 15, 2016 11:00 SISSA, Room 128 |
G. Mussardo |
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Prime Suspects and Coprime Accomplices: Quantum Tales in Number Theory | ||
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📅 🕓 🚪 |
November 8, 2016 11:00 SISSA, Room 128 |
E. Tartaglia |
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Logarithmic minimal models with Robin boundary conditions | ||
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📅 🕓 🚪 |
October 11, 2016 11:00 SISSA, Room 128 |
M. Mintchev |
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Non-equilibrium quantum transport: quantum heat engines and full counting statistics | ||
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📅 🕓 🚪 |
October 5, 2016 11:00 SISSA, Room 128 |
Huan-Qiang Zhou |
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Fidelity mechanics: analogues of four thermodynamic laws and Landauer’s principle |
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October 4, 2016 11:00 SISSA, Room 005 |
M. Batchelor |
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Free parafermions |
📅 🕓 🚪 |
July 15, 2016 11:00 SISSA, Room 128 |
Z. Zimboras |
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Negativity in free fermion systems | ||
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📅 🕓 🚪 |
July 12, 2016 11:00 SISSA, Room 128 |
V. Eisler |
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Universal front propagation in the XY spin chain with domain wall initial conditions | ||
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📅 🕓 🚪 |
June 28, 2016 11:00 SISSA, Room 128 |
B. Poszgay |
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Quantum quenches and exact correlations in the Heisenberg spin chains | ||
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📅 🕓 🚪 |
June 17, 2016 11:00 SISSA |
A. Lode |
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Fragmentation and correlations of interacting ultracold multicomponent bosons |
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May 26, 2016 11:30 ICTP |
F. Marquardt |
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Light, sound and topology |
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May 24, 2016 11:00 ICTP |
E. Dalla Torre |
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Parametric resonances: from single atoms to many-body systems |
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May 19, 2016 11:00 SISSA, Room 005 |
A. Jakovac |
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Functional renormalization group in fermionic systems | ||
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📅 🕓 🚪 |
May 10, 2016 11:30 SISSA, Room 005 |
R. Egger |
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Multichannel Kondo dynamics and Surface Code from Majorana bound states | ||
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May 6, 2016 11:30 SISSA, Room 005 |
A. Fring |
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Non-Hermitian quasi-exactly solvable models of E_{2} Lie algebraic type | ||
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May 5, 2016 14:30 ICTP, Stasi Room |
U. Schneider |
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May 3, 2016 11:00 SISSA, Room 005 |
F. Bouchet |
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Large deviation theory applied to climate physics, a new frontier of statistical physics | ||
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April 28, 2016 11:30 ICTP, Stasi Room |
E. Collini |
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April 28, 2016 11:00 SISSA, Room 005 |
O.A. Castro-Alvaredo |
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Measures of entanglement from quantum field theory methods | ||
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April 26, 2016 11:00 SISSA, Room 005 |
B. Doyon |
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Non-equilibrium energy transport at quantum criticality | ||
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April 22, 2016 14:00 SISSA, Room 005 |
M. Polini |
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Hydrodynamic transport, laminar flow, and the AdS/CFT viscosity bound in a graphene field effect transistor | ||
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April 20, 2016 15:30 ICTP, Stasi Room |
A. Varlamov |
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April 15, 2016 14:30 SISSA, Room 005 |
J.M. Stephan |
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Entanglement evolution after inhomogeneous quantum quenches, and the arctic circle | ||
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April 14, 2016 11:30 ICTP, Stasi Room |
A.K. Heidelberg |
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April 12, 2016 11:00 SISSA, Room 005 |
J. Dubail |
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Inhomogeneous quantum systems in 1d: how does one describe them with Conformal Field Theory? | ||
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March 31, 2016 11:30 ICTP, Stasi Room |
E. Vesselli |
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March 22, 2016 11:00 SISSA, Room 128 |
J. Kurchan |
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Darwinian versus thermal optimization | ||
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March 18, 2016 15:00 SISSA, Room 005 |
R. Sinha |
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Thermalization with Chemical Potentials, and Higher Spin Black Holes | ||
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March 17, 2016 11:30 ICTP, Stasi Room |
D. Fausti |
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March 15, 2016 11:30 SISSA, Room 005 |
S. Diehl |
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Universal Quantum Physics in Driven Open Many-Body Systems | ||
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March 8, 2016 11:00 SISSA, Room 005 |
G. Sierra |
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Entanglement over the Rainbow | ||
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March 3, 2016 14:00 SISSA, Room 128 |
C. Maes |
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Driving-induced stability with long-range effects | ||
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February 22, 2016 14:30 ICTP, Stasi Room |
M. Kruger |
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Fluctuation Induced Interactions In and Out of Equilibrium | ||
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February 17, 2016 15:00 ICTP |
I. Carusotto |
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February 16, 2016 11:00 SISSA, Room 128 |
W. Krauth |
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Fast Irreversible Monte Carlo simulations beyond the Metropolis paradigm: Applications to interacting particles and to spin systems | ||
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February 9, 2016 11:00 SISSA, Room 128 |
T. Fokkema |
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Supersymmetric lattice models: the field theory connection | ||
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February 2, 2016 11:00 SISSA, Room 128 |
A. Chiocchetta |
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Short-time universality and aging in isolated quantum systems | ||
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January 26, 2016 11:00 SISSA, Room 128 |
F. Corberi |
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Condensation of large fluctuations in a statistical system |