Onset of many-body quantum chaos due to breaking integrability

TL;DR

This paper models the onset of many-body quantum chaos as a Fock-space delocalization process, analyzing how integrability-breaking perturbations induce chaos and thermalization in quantum systems, with size-dependent scaling and intermediate regimes.

Contribution

It introduces a Fock-space delocalization framework to describe chaos onset, providing analytical estimates and numerical results for size scaling and intermediate regimes.

Findings

Chaos onset scales to zero with system size.

Intermediate nonchaotic regime exists with partial resonances.

Transition between chaotic regimes depends on system size and mean free path.

Abstract

Integrable quantum systems of finite size are generically robust against weak enough integrability-breaking perturbations, but become quantum chaotic and thermalizing if the integrability-breaking is strong enough. We argue that the onset of quantum chaos can be described as a Fock-space delocalization process, with the eigenstates of the integrable system being taken as the "Fock states". The integrability-breaking perturbation introduces hopping in this Fock space, and chaos sets in when this hopping delocalizes the many-body eigenstates in this space. Depending on the range of the dominant Fock-space hopping, delocalization can occur either through a crossover, or via a transition that becomes sharp in the appropriate large-system dynamic limit. In either case, the perturbation strength at the onset of chaos scales to zero in the usual thermodynamic limit, with a size-dependence that we estimate analytically and compute numerically for a few specific models. We also identify two intermediate finite-size-dependent regimes: There is generally an intermediate nonchaotic regime in which integrability is broken strongly enough to produce some system-wide many-body resonances but not enough to thermalize the system. In spatially extended systems (but not in quantum dots) there is also a crossover or transition between chaotic regimes where the ratio of the system size to the mean free path of the quasiparticles of the integrable system is small versus large compared to unity.