Non-post-selection entanglement concentration by ordinary linear optical devices

Two remote parties, Alice and Bob initially share some non-maximally entangled states. Through the entanglement concentration by local operation and classical communication(LOCC), they may obtain an outcome of maximally entangled states in the price of decreasing the number of pairs shared by them. Recently, entanglement concentration have been experimently demonstrated by post-selection (T. Yamamoto et al, Nature, 421, 343(2003) and Z. Zhao et al, quant-ph/0301118). Here we give a modified scheme which can be used for the entanglement concentration without any post selection by using only practically existing linear optical devices. In particular, a sophisticated photon detector to distinguish one photon or two photons is not required. Our scheme can be used to really produce the event-ready maximally entangled pairs through LOCC provided that the requested raw pairs are supplied deterministically. A detailed experimental plan with spontaneous parametric down conversion(SPDC) is shown.