A cavity-mediated quantum CPHASE gate between NV spin qubits in diamond
  While long spin coherence times and efficient single-qubit quantum control have been implemented successfully in nitrogen-vacancy (NV) centers in diamond, the controlled coupling of remote NV spin qubits remains challenging. Here, we propose and analyze a controlled-phase (CPHASE) gate for the spins of two NV centers embedded in a common optical cavity and driven by two off-resonant lasers. In combination with previously demonstrated single-qubit gates, CPHASE allows for arbitrary quantum computations. The coupling of the NV spin to the cavity mode is based upon Raman transitions via the NV excited states and can be controlled with the laser intensities and relative phase. We find characteristic laser frequencies at which the scattering amplitude of a laser photon into the cavity mode is strongly dependent on the NV center spin. A scattered photon can be reabsorbed by another selectively driven NV center and generate a conditional phase (CPHASE) gate. Gate times around 200 ns are within reach, nearly three orders of magnitude shorter than typical NV spin coherence times of around 10 microseconds. The separation between the two interacting NV centers is only limited by the extension of the cavity. 
Guido Burkard, V. O. Shkolnikov, D. D. Awschalom
cond-mat.mes-hall quant-ph