A 2-PHOTON INTERFERENCE EXPERIMENT USING TYPE-II OPTICAL PARAMETRIC DOWN-CONVERSION

A pair of orthogonally polarized light quanta, produced from type II parametric down conversion, is injected into a single input port of a beamsplitter and detected coincidently by two detectors placed in the two output ports of the beamsplitter. With the help of a set of crystal quartz plates, a Pockel's cell and two linear polarizers, the coincidence counting rate exhibits interference modulation of the pump frequency with 88% visibility when manipulating the voltage across the Pockel's cell, regardless of the optical delay (which is much greater than the coherence length) and the mutual incoherence. This two-photon interference effect is responsible by a nonclassical two-photon state which is entangled both in spin and in space-time.