  We experimentally demonstrate synchronization between two distinct ensembles
of cold atoms undergoing steady state superradiance within a single
longitudinal and transverse mode of the same optical cavity. The
synchronization process is studied first in terms of the time dynamics of
re-synchronization when the phase alignment of the two oscillators is abruptly
broken. We also observe the steady state behavior of the lasers as their
relative frequency is continuously varied. This system has the potential to
realize a non-equilibrium quantum phase transition and could inform future
implementations of milliHertz linewidth lasers.
