  We demonstrate the coherent coupling and the resulting transfer of phase
information between microwave and optical fields in a single nitrogen vacancy
center in diamond. The relative phase of two microwave fields is encoded in a
coherent superposition spin state. This phase information is then retrieved
with a pair of optical fields. A related process is also used for the transfer
of phase information from optical to microwave fields. These studies show the
essential role of dark states, including optical pumping into the dark states,
in the coherent microwave-optical coupling and open the door to the full
quantum state transfer between microwave and optical fields in a solid-state
spin ensemble.
