Generation of entangled photon states by using linear optical elements

We present a scheme to generate the polarization-entangled two-photon state 1/root2(\H>\V>+parallel toV>\H>), which is of much interest in the field of quantum information processing. Furthermore, we demonstrate the capability of this concept in respect of a generalization to entangle N-photon states for interferometry and lithography. This scheme requires single-photon sources, linear optical elements, and a multifold coincidence detection.