Single-qubit-assisted entanglement concentration of partially entangled multi-electron spin W states

We propose an entanglement concentration protocol (ECP) for nonlocal N-electron systems in a partially entangled W state, resorting to ancillary single electrons and charge detection. Compared with other ECPs for W states, our ECP has some advantages. Firstly, only an entangled N-electron system is required in each round of entanglement concentration, not two copies. Secondly, only one of the users, say Charlie, needs to perform the protocol, while all parties should perform the same operations as Charlie in other ECPs for W-class states. Thirdly, only Charlie asks other parties to retain or discard their electrons, and they do not need to check their measurement results, which greatly simplifies the complication of classical communication. Fourthly, our ECP has a higher success probability than other ECPs for W-class states as the parties can recycle their concentration when they obtain a system in another W-class state with less entanglement. These advantages make our ECP more feasible for practical applications.