Complex and Disordered Quantum Systems

  • PIs: Scardicchio

The combined effects of strong interactions and disorder presents one of the most exciting and challenging fields of modern quantum condensed matter physics. Attempts to understand interacting disordered quantum systems in terms of their excitations, energy and particle transport, relaxation dynamics and thermalization have been the focus of much attention recently.

In quantum systems, strong disorder may lead to non-ergodicity and entail permanent out-of-equilibrium and glassy behavior for two rather different reasons:

  • localization in Hilbert space due to many-body versions of Anderson localization;
  • confinement in configuration space due to high barriers arising from frustration between competing interactions, such as in quantum spin glasses.

A unified understanding of the underlying mechanisms and the phenomenology of the entailing quantum glassiness is not yet available, but it is starting to emerge.

A particularly interesting question concerns the problem of understanding how such non-ergodicity arises close to disorder-tuned quantum phase transitions between conducting and insulating phases. Recent results suggest a rich phenomenology of spatial and spectral properties of the relevant excitations and ground states.

We address the interplay of interactions, glassiness, localization, and superfluidity from many different angles. On one hand, we are studying models of quantum glasses and their collective low energy properties. On the other hand, we try to understand the transition from non-ergodic to ergodic quantum dynamics in strongly disordered many body systems.

Disordered quantum systems are also interesting objects of study from the point of view of complexity and quantum information theory. Some of the arising conceptual questions are: are many-body wavefunctions of quantum non-ergodic systems less complex than those of ergodic systems, and can this be used for efficient quasiparticle representations of non-ergodic, localized systems?

Can classically hard problems be more easily solved by quantum annealing?

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