Observation of Simultaneity in Parametric Production of Optical Photon Pairs

The quantum mechanical description of parametric fluorescence is the splitting of a single photon into two photons. This description has been verified by observing coincidences between photons emitted by an ammonium dihydrogen phosphate crystal pumped by a 325-nm He-Cd laser. The coincidence rate R₂ decreases to the calculated accidental rate <0. 03R₂ (max), unless the two detectors are arranged to satisfy energy and momentum conservation and have equal time delays.