Entanglement concentration of unknown states on separate nitrogen-vacancy centers via error-detected entanglement swapping

Here we propose a new entanglement concentration protocol (ECP) for unknown less-entangled states on separate nitrogen-vacancy (NV) centers. The protocol exploits the error-detected entanglement swapping based on cavity quantum electrodynamics, which is more robust and practical than previous ECPs for NV centers. We present in detail the principle and the advantages of the ECP. Also, the performance of the protocol with current experimental parameters is discussed. We believe that the ECP could provide a promising building block for solid-state quantum repeaters and quantum networks in the future.