  We report the observation of the vacuum Rabi splitting of a single quantum
emitter by measuring its direct spontaneous emission into free space. We used a
semiconductor quantum dot inside a photonic crystal nanocavity, in conjunction
with an appropriate cavity design and filtering with a polarizer and an
aperture, enabling the extraction of the inherently-weak emitter's signal. The
emitter's vacuum Rabi spectra exhibit clear differences to those measured by
detecting the cavity photon leakage. Moreover, we observed an asymmetric vacuum
Rabi spectrum induced by interference between the emitter and cavity detection
channels. Our observations lay the groundwork for accessing various cavity
quantum electrodynamics phenomena that manifest themselves only in the
emitter's direct spontaneous emission.
