Continuously monitored ergodicity breaking in multimode cavities

We present experimental evidence for replica symmetry breaking in a system of effective spins coupled to a multimode optical cavity. This phenomenon is observed experimentally by exploiting a correspondence between individual trajectories of a continuously monitored system and replicas in the analysis of spin glasses. We experimentally characterize the glassy phase, by analyzing the landscape of its minima, which show signatures of ultrametricity. The landscape of minima is qualitatively similar to that of infinite-range spin glasses in equilibrium, but exhibits quantitative differences, reflecting the fact that the relaxation dynamics in a cavity is inherently nonequilibrium. We present experimental and numerical evidence that this nonequilibrium dynamics enhances the stability of minima in the spin glass phase relative to equilibrium Metropolis dynamics: thus, the spin glass acts as a stable memory. We discuss the possible theoretical origins of this memory enhancement.