Spontaneous parametric down-conversion (also known as SPDC, parametric fluorescence or parametric scattering) is a nonlinear instant optical process that converts one photon of higher energy (namely, a pump photon) into a pair of photons (namely, signal and idler photons) of lower energy, in accordance with the laws of energy conservation and momentum conservation. It is an important process in quantum optics, for the generation of entangled photon pairs and of single photons.
