Quantum many body scars as remnants of unusually stable periodic many-body orbits

by Keita Omiya (PSI)

Tuesday 10 Sept 2024, 11:00 → 12:00 Europe/Zurich

WHGA/121

Quantum many-body scars (QMBS) represent a weak ergodicity-breaking phenomenon that defies the common scenario of thermalization in closed quantum systems. In scarred systems, long-lasting oscillations are typically observed, but only for special initial states. QMBS are often regarded as a many-body analog of quantum scars (QS)---a single-particle phenomenon in quantum chaos---due to their superficial similarities, such as sustained oscillatory behavior emanating from special eigenstates. However, unlike QS, which are intimately linked to periodic orbits with classically chaotic systems, a clear connection between QMBS and classical chaos has remained elusive.

It has nevertheless been speculated that in an appropriate semiclassical limit, long-lasting oscillations in QMBS should have a similar correspondence to (weakly unstable) periodic orbits in classically chaotic systems. In this seminar, I will present a counterexample to this conjecture by studying a bosonic model that is tractable in the limit of a large number of flavors. The dynamics of out-of-time-ordered correlators (OTOCs), a standard measure of quantum chaos, suggest that QMBS do not display chaotic behavior in the semiclassical limit. In contrast, chaotic dynamics are expected for initial states that are not associated with QMBS. Interestingly, the anomalous OTOC dynamics persist even under weak perturbations that eliminate the scarred eigenstates, suggesting a certain robustness in the phenomenon.