Genuine quantum scars in many-body spin systems

TL;DR

This paper reveals that many-body quantum systems can exhibit quantum scars—special eigenstates that retain memory of initial conditions—despite being chaotic and thermalizing, thus challenging the expectation of complete ergodicity.

Contribution

It demonstrates the widespread presence of quantum scars in many-body spin systems, showing they can weakly break ergodicity even in thermalized, chaotic regimes.

Findings

Quantum scars are prevalent in many-body spin models.

Scarring causes systems to retain memory of initial states.

Quantum scars coexist with thermalization and chaos.

Abstract

Chaos makes isolated systems of many interacting particles quickly thermalize and forget about their past. Here, we show that quantum mechanics hinders chaos in many-body systems: although the quantum eigenstates are thermal and strongly entangled, exponentially many of them are scarred, that is, have an enlarged weight along underlying classical unstable periodic orbits. Scarring makes the system more likely to be found on an orbit it was initialized on, retaining a memory of its past and thus weakly breaking ergodicity, even at long times and despite the system being fully thermal and the eigenstate thermalization hypothesis fulfilled. We demonstrate the ubiquity of quantum scarring in many-body systems by considering a large family of spin models, including some of the most popular ones from condensed matter physics. Our findings, at hand for modern quantum simulators, prove structure in spite of chaos in many-body quantum systems.