10th Trieste–Ljubljana meeting @ SISSA

When: Monday, 23rd Oct, 2023
Where: SISSA – Trieste

10:00–10:15 Welcome coffee & opening remarks
10:15–10:45 Colin Rylands (SISSA)
10:50–11:20 Chiara Paletta (FMF)
11:25–11:55 Poetri Tarabunga (ICTP)
12:00–15:15 Lunch break and Discussion
15:15–15:45 Miroslav Hopjan (IJS)
15:50–16:20 Alessio Paviglianiti (SISSA)
16:25–16:55 Gerald E. Fux (ICTP)
16:55– Closing remarks & discussions
Quantum Mpemba effect in integrable models
Speaker Colin Rylands
Abstract Non-equilibrium systems can exhibit surprising properties when compared to their equilib- rium counterparts. One such example, for classical systems, is the Mpemba effect which describes the situation where non equilibrium states can relax to equilibrium faster the further, they are from equilibrium initially. In this talk, I will describe an analogous effect in closed many body quantum systems which have a conserved U(1) charge: in certain cases, a system prepared in an initially asymmetric state can restore the symmetry and therefore relax faster than a more symmetric one. I will discuss how this effect is identified in integrable models and derive a generic set of criteria for its occurrence, which depend on properties of the initial state. Using some specific examples, I will then show situations where this effect occurs or does not and how this can be explained by examining fluctuations of the charge in the initial state.
Hidden strong symmetries in a range 3 deformation of the Hubbard model
Speaker Chiara Paletta
Abstract We examine a spin chain representing an open quantum system governed by the Lindblad equation, with external driving acting in the bulk. This model corresponds to a new integrable range 3 elliptic deformation of the Hubbard model. We show the appearance of multiple nonequi- librium steady states (NESS): the system retains memory of the initial state, even though the well-known invariance under spin su(2) and charge su(2) transformations of the Hubbard Hamil- tonian is broken. We motivate this phenomenon by the existence of hidden strong symmetries in the form of quasi-local operators. Based on the works 2301.01612 and 2305.01922 with M. de Leeuw, B. Pozsgay and E. Vernier.
Magic in many-body systems
Speaker Poetri Tarabunga
Abstract Non-stabilizerness—commonly known as magic—measures the extent to which a quantum state deviates from stabilizer states and is a fundamental resource for achieving universal quan- tum computation. In recent years, there has been an increasing interest in understanding the role of magic in quantum many-body systems. This is not an easy task, as magic is a noto- riously difficult quantity to compute, especially in large systems. In this talk, I will present recent advances in the investigation of magic in many-body systems. Specifically, I will discuss recently developed numerical methods to compute the magic, as quantified by stabilizer Renyi entropy and mana, two measures of magic that are relatively simple to compute. Furthermore, I will discuss the findings on the behavior of many-body magic, which have been made possible through the application of these methods.
Scale-invariant critical dynamics at eigenstate transitions
Speaker Miroslav Hopjan
Abstract The notion of scale invariant dynamics is well established at late times in quantum chaotic systems, as illustrated by the emergence of a ramp in the spectral form factor (SFF). Building on recent results [1,2,3], we explore features of scale invariant dynamics of SFF and survival probability at criticality, i.e., at eigenstate transitions from quantum chaos to localization. We show that, in contrast to the quantum chaotic regime, the quantum dynamics at criticality do not only exhibit scale invariance at late times, but also at much shorter times that we refer to as mid-time dynamics [4]. Our results apply to both quadratic and interacting models. Specifically, we study Anderson models in dimensions three to five and power-law random banded matrices for the former, and the quantum sun model and the ultrametric model for the latter. Based on empirical comparisons, we discuss universal trends in features of the scale-invariant critical dynamics, which are expressed by smooth functions of a tuning parameter [4].

[1] J. Šuntajs, T. Prosen, L. Vidmar, Ann. Phys. 435, 168469 (2021).
[2] J. Šuntajs and L. Vidmar, Phys. Rev. Lett. 129, 060602 (2022).
[3] M.Hopjan and L. Vidmar, Phys. Rev. Lett. 131, 060404 (2023).
[4] M.Hopjan and L. Vidmar, arXiv:2309.16005 (2023).

Multipartite Entanglement in the Measurement-Induced Phase Transition of the Quantum Ising Chain
Speaker Alessio Paviglianiti
Abstract External monitoring of quantum many-body systems can give rise to a measurement- induced phase transition, characterized by a change in behavior of the entanglement entropy from an area law to an unbounded growth. We show that this transition extends beyond bipartite cor- relations to multipartite entanglement. Using the quantum Fisher information, we investigate the entanglement dynamics of a continuously monitored quan- tum Ising chain. Multipartite entanglement exhibits the same phase boundaries observed for the entropy in the post-selected no-click trajectory. Instead, quantum jumps give rise to a more complex behavior that still features the transition, but adds the possibility of having a third phase with logarithmic entropy but bounded multipartiteness.

Measurement Induced Phase Transition in Magic
Speaker Gerald E. Fux
Abstract The robustness of the volume law entanglement phase in hybrid quantum circuits against a finite measurement rate is of special interest because entanglement constitutes an important resource for quantum computation. However, to achieve a quantum advantage over classical computation not only entanglement but also so-called “magic” is a necessary resource. Magic describes the distance between a quantum circuit and its closest stabilizer circuit that can be simulated classically with polynomial resources. In this talk we present a hybrid quantum cir- cuit setup that exhibits a measurement induced phase transition in magic [1] different from the known entanglement phase transition.

[1] G. E. Fux, E. Tirrito, M. Dalmonte, R. Fazio, in preparation

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