Four-photon entanglement from down-conversion - art. no. 010102

Double-pair emission from type-II parametric down-conversion results in a h ighly entangled four-photon state. Due to interference, which is similar to bunching from thermal emission, this state is not simply a product of two pairs. The observation of this state can be achieved by splitting the two e mission modes at beam splitters and subsequent detection of a photon in eac h output. Here we describe the feature's of this state and construct a Bell -type inequality, which gives a necessary and sufficient condition for a fo ur-photon test of local realistic hidden variable theories.