  Spontaneous two photon emission from a solid-state single quantum emitter is
observed. We investigated photoluminescence from the neutral biexciton in a
single semiconductor quantum dot coupled with a high Q photonic crystal
nanocavity. When the cavity is resonant to the half energy of the biexciton,
the strong vacuum field in the cavity inspires the biexciton to simultaneously
emit two photons into the mode, resulting in clear emission enhancement of the
mode. Meanwhile, suppression was observed of other single photon emission from
the biexciton, as the two photon emission process becomes faster than the
others at the resonance.
