We demonstrate a quantum optical switch based on an individual Silicon
Vacancy (SiV) color center integrated in a diamond nanophotonic device.
Individual SiV centers with excellent optical coherence properties are
introduced nearly deterministically at the center of a 1D diamond
photonic crystal cavity via targeted Si ion implantation. We observe
~40% attenuation of weak probe transmission through the coupled system
that saturates at a level of an individual photon per system's
bandwidth. The transmission is controlled by switching the metastable
state of an individual SiV defect. Photon correlation measurements are
used to verify optical switching at the single photon level.