Detecting Single Microwave Photons with NV Centers in Diamond.

We propose a scheme for detecting single microwave photons using dipole-induced transparency (DIT) in an optical cavity resonantly coupled to a spin-selective transition of a negatively charged nitrogen-vacancy (NV) defect in diamond crystal lattices. In this scheme, the microwave photons control the interaction of the optical cavity with the NV center by addressing the spin state of the defect. The spin, in turn, is measured with high fidelity by counting the number of reflected photons when the cavity is probed by resonant laser light.