  We report on the first experimental observation of spontaneous mirror
symmetry breaking (SSB) in coherently driven-dissipative coupled optical
cavities. SSB is observed as the breaking of the spatial or mirror Z2 symmetry
between two symmetrically pumped and evanescently coupled photonic crystal
nanocavities, and manifests itself as random intensity localization in one of
the two cavities. We show that, in a system featuring repulsive boson
interactions (U > 0), the observation of a pure pitchfork bifurcation requires
negative photon hopping energies (J < 0), which we have realized in our
photonic crystal molecule. SSB is observed over a wide range of the
two-dimensional parameter space of driving intensity and detuning, where we
also find a region that exhibits bistable symmetric behavior. Our results pave
the way for the experimental study of limit cycles and deterministic chaos
arising from SSB, as well as the study of nonclassical photon correlations
close to SSB transitions.
