Illustration: Alan Stonebraker

Figure 4: In each cavity is a diamond with an $NV-$ center in a specific state, the two tuned to have identical excitation energies. Both systems are exposed to a weak laser pulse that, on average, will achieve one excitation. The single $NV-$ center excited will emit a photon that, after passing though beam splitters and an interferometer, is detected without giving information as to which system was excited. This leaves the two $NV-$ centers entangled.