We present two three-photon entanglement concentration protocols (ECPs) for an arbitrary unknown less-entangled W-class state, resorting to linear optical elements. In our normal ECP, the three parties can obtain a three-photon system in a standard W state probabilistically, similar to the Schmidt projection method. In our improved ECP, the parties obtain not only some higher-fidelity partially entangled three-photon systems but also some entangled two-photon systems in each round of concentration with two copies of three-photon systems. It is interesting to show that the three-photon and two-photon systems kept after a round of concentration have the same parameters. The parties can obtain some three-photon systems in a standard W state with a far higher success probability than the normal ECP, by exploiting the three-photon and two-photon systems with the same parameters as the resource for the next round of concentration. Both of these ECPs may have good applications in quantum communication in the future.
We present two two-step practical entanglement concentration protocols for concentrating an arbitrary three-particle less-entangled
W state into a maximally entangled
W state assisted with single photons. The first protocol uses the linear optics and the second protocol adopts the cross--Kerr nonlinearity to perform the protocol. In the first protocol, based on the postselection principle, three parties say Alice, Bob, and Charlie in different distant locations can obtain the maximally entangled
W state from the arbitrary less-entangled
W state with a certain success probability. In the second protocol, the parties are not required to possess the sophisticated single-photon detectors and the concentrated photon pair can be retained after performing this protocol successfully. Moreover, the second protocol can be repeated to get a higher success probability. Both protocols may be useful in practical quantum-information applications.