Perfect oscillatory dynamics from quantum many-body scars

The concept of quantum many-body scars -- atypical, nonergodic energy eigenstates of an otherwise ergodic many-body system -- was recently introduced to explain the surprising long-lived oscillations observed in an interacting, constrained spin chain following a quantum quench. Here, we provide numerical evidence that a suitable, quasi-local deformation of the system leads to a dramatic increase of the lifetime of the oscillations, even possibly allowing them to last for an indefinitely long time. We show that this seemingly perfect oscillatory dynamics can be understood via an emergent large SU(2)-spin undergoing precession, contained within a special subspace of the many-body Hilbert space, while the rest of the system remains ergodic. The presence of such dynamics severely constrains the structure of certain energy eigenstates in the thermodynamic limit. Furthermore, we introduce a toy model which captures the salient features of quantum many-body scarring.