  We experimentally observe a spontaneous temporal symmetry breaking
instability in a coherently-driven passive optical Kerr resonator. The cavity
is synchronously pumped by time-symmetric pulses yet we report output pulses
with strongly asymmetric temporal and spectral intensity profiles, with up to
71% of the energy on the same side of the pump center frequency. The
instability occurs above a certain pump power threshold but remarkably vanishes
above a second threshold, in excellent agreement with theory. We also observe a
generalized bistability in which an asymmetric output state coexists with a
symmetric one in the same pumping conditions.
