Metastable discrete time-crystal resonances in a dissipative central spin system

We consider the nonequilibrium behavior of a central spin system where the central spin is periodically reset to its ground state. The quantum-mechanical evolution under this effectively dissipative dynamics is described by a discrete-time quantum map. Despite its simplicity this problem shows surprisingly complex dynamical features. In particular, we identify several metastable time-crystal resonances. Here the system does not relax rapidly to a stationary state but undergoes long-lived oscillations with a period that is an integer multiple of the reset period. At these resonances the evolution becomes restricted to a low-dimensional state space within which the system undergoes a periodic motion. Generalizing the theory of metastability in open quantum systems, we develop an effective description for the evolution within this long-lived metastable subspace and show that in the long-time limit a nonequilibrium stationary state is approached. Our study links to timely questions concerning emergent collective behavior in the "prethermal"stage of a dissipative quantum many-body evolution and may establish an intriguing link to the phenomenon of quantum synchronization.