Entanglement purification for memory nodes in a quantum network

The entanglement between quantum memory nodes is a prerequisite in a quantum network, and the diamond nitrogen-vacancy (NV) center is a promising candidate serving as a quantum memory node. Here, we investigate the possibility of achieving an entanglement purification protocol (EPP) for entangled NV centers in distant diamonds. To construct the EPP, we design a nondestructive parity-check detector (PCD) utilizing an auxiliary polarization-entangled photon pair, which makes our EPP less time consuming and insensitive to the phase fluctuation of the optical path length. The satisfied fidelity of an NV center pair after purification and efficiency of obtaining a purified NV center pair with our EPP can be obtained with current experimental techniques in the realistic condition. This EPP is useful for a quantum network in which NV centers are used as quantum memory nodes.