  We propose a system for observing the correlated phase dynamics of two
mesoscopic ensembles of atoms through their collective coupling to an optical
cavity. We find a dynamical quantum phase transition induced by pump noise and
cavity output-coupling. The spectral properties of the superradiant light
emitted from the cavity show that at a critical pump rate the system undergoes
a transition from the independent behavior of two disparate oscillators to the
phase-locking that is the signature of quantum synchronization.
