Entanglement distillation for W class states

In this paper, we first propose a general entanglement distillation protocol for three-particle W class state, which can concentrate the state (non-maximally entangled W state). The general protocol is mainly based on the unitary transformations on the auxiliary particles and the entangled particles, and a feasible physical scheme is suggested based on the cavity QED techniques. The protocol and scheme can be extended to the entanglement concentration of N-particle case.