Observation of metastability in open quantum dynamics of a solid-state system.

Metastability is a ubiquitous phenomenon in nonequilibrium physics and classical stochastic dynamics. It arises when the system dynamics settles in long-lived states before eventually decaying to true equilibria. Remarkably, it has been predicted that quantum metastability can also occur in continuous- and discrete-time open quantum dynamics. However, the direct experimental observation of metastability in open quantum systems has remained elusive. Here, we experimentally observe metastability in the discrete-time evolution of a single nuclear spin in diamond, realized by sequential Ramsey interferometry measurements (RIMs) of a nearby nitrogen-vacancy electron spin. We demonstrate that the metastable polarization of the nuclear spin emerges at a metastable region of measurement times determined by the spectral structure of the quantum channel induced by the RIM. Such metastable polarization enables high-fidelity single-shot readout of the nuclear spin and the observation of an ultralong spin relaxation time of more than 10 s at room temperature. Our results represent a concrete step towards uncovering nonequilibrium physics in open quantum dynamics, which is practically relevant for the utilization of metastable information for various quantum information processing tasks.