Nonlocal entanglement concentration of separate nitrogen-vacancy centers coupling to microtoroidal resonators

Here we propose two practical protocols to concentrate entanglement between separate nitrogen-vacancy (N-V) centers in less entangled state via coupling to microtoroidal resonators. We construct the parity check gate of the N-V center and microtoroidal resonator systems via the interaction with the ancillary photon input-output process near the microtoroidal resonators. Thus the parity of the N-V center state can be readout by the measurement on the ancillary photon. Then we introduce the parity check operations to entanglement concentration protocols. Considering current techniques, we also discuss the feasibility of our proposal and its experimental challenges.