  We propose an efficient scheme for a coherent quantum interface between
microwave and optical photons using nitrogen-vacancy (NV) centers in diamond.
In this setup, an NV center ensemble is simultaneously coupled to an optical
and a microwave cavity. We show that, by using the collective spin excitation
modes as an intermediary, quantum states can be transferred between the
microwave cavity and the optical cavity through either a double-swap scheme or
a dark-state protocol. This hybrid quantum interface may provide interesting
applications in single microwave photon detections or quantum information
processing.
