Transfer of Phase Information between Optical and Microwave Fields via an Electron Spin

Advances in quantum information processing using superconducting circuits have stimulated strong interest in developing interfaces between microwave and optical fields. While microwave fields can enable on-chip or local communications between superconducting qubits, optical fields remain the ideal choice for off-chip or long distance communications. Interfaces between microwave and optical fields via their coupling to a spin ensemble have been proposed. Here we demonstrate that a single nitrogen vacancy (NV) center in diamond can enable the coherent coupling between microwave and optical fields. We map the relative phase of two microwave fields to a spin coherence in a NV center and then read out the phase using two optical fields via a dark-state based process. A closely related process is also used for the transfer of phase information from optical to microwave fields. Similar coherent coupling in a NV ensemble can enable full quantum-state transfer between microwave and optical fields.