Abstract
Recently the question of whether an isolated quantum system can reach thermal equilibrium under its own dynamics has attracted substantial interest. This question is crucial because it is closely related to the ergodic hypothesis, which is one of the fundamental hypotheses of statistical mechanics. Previously it was believed that ergodicity breaking can only occur in special integrable models, which are fragile to even weak perturbations. However, our understanding of this question has been revolutionized in the past decade, and two distinct paths of ergodicity breaking have been identified. On the one hand, it was found that strong disorders provide a robust mechanism for ergodicity breaking in one-dimensional systems, leading to the phenomenon of many-body localization. This mechanism is considered a form of strong ergodicity breaking because all eigenstates in the energy spectrum become nonthermal. As a result, strong revivals can be expected in quench dynamics starting from almost all initial states. On the other hand, a form of weak ergodicity breaking has also been identified, which is characterized by the fact that the quench dynamics from special initial states exhibits persistent revivals, whereas the quench dynamics from almost all other initial states displays much faster relaxations. This phenomenon, known as quantum many-body scarring, usually occurs in a clean system, and provides interesting new insights into the dynamics of an isolated quantum system.