Quantum many-body scarring: weak ergodicity breaking in an interacting Rydberg atom array simulator

A central postulate of statistical mechanics is that of ergodicity -- a generic state prepared out of equilibrium is believed to explore its allowed phase phase and eventually thermalize. Recently, quench experiments in an interacting Rydberg atom array [Nature 551, 579 (2017)] demonstrated interesting nonequilibrium dynamics of a new kind: surprising periodic revivals and a lack of thermalization from certain simple initial states, while quick relaxation and equilibriation from others. Here we show that these observations are attributed to the presence of a small number of exceptional, nonthermal many-body eigenstates dubbed ``quantum many-body scars" that violate the eigenstate thermalization hypothesis. Furthermore, underlying this behavior is an isolated periodic orbit captured in a suitable ``semiclassical" analysis using matrix product states, which suggest a connection to scars in single-particle chaotic systems. Lastly we present work uncovering a nearby parent Hamiltonian that hosts perfect many-body scars, and construct a toy model with similar phenomenology. Quantum many-body scarring represents a new class of quantum dynamics in strongly interacting systems resulting from a weak form of ergodicity breaking, with direct experimental signatures.