Practical scheme for entanglement concentration

We present a realistic purification scheme for pure nonmaximally entangled states. In the scheme, two distant parties Alice and Bob first start with two shared but less entangled photon pairs to produce a conditional four-photon Greenberger-Horne-Zeilinger state, and then perform a 45 degrees polarization measurement onto one of the two photons at each location such that the remaining two photons are projected onto a maximally entangled state.