Producing the event ready two photon polarization EPR state with linear optics devices

We propose a scheme to produce the maximally two photon polarization entangled state(EPR state) with single photon sources and the linear optics devices. In particular, our scheme only requires the photon detectors used there to distinguish the vacuum and non-vacuum Fock number states. A sophisticated photon detector distinguishing one-photon state and two-photon state is unnecessary. The resource of maximally entangled state(EPR state) plays a fundamentally important role in testing the quantum laws related to the nolocality [1] and in many application in quantum information such as the quantum teleportation [2,3], quantum key distribution [4] and quantum computation [5]. So far, it is generally believed that the two photon polarized EPR state is particularly useful in quantum information processing. The observation of EPR pairs has been carried out by many experiments (for example, ref [6]). However, those polarized photon pairs were only produced randomly since there is no way to know whether a polarization EPR pair is generated without destroying the state itself. The post selection property in the EPR state generation is not crucial to some quantum tasks such as the testing of the violation of Bell inequality. However, in many