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 $R_C$ decreases to the calculated accidental rate [$<0.03\mathrm{}{R}_C\left(max\right)$], unless the two detectors are arranged to satisfy energy and momentum conservation and have equal time delays.