Figure 4: In each cavity is a diamond with an ${\text{NV}}^{-}$ 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 ${\text{NV}}^{-}$ 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 ${\text{NV}}^{-}$ centers entangled.