Spontaneous parametric down conversion

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Spontaneous parametric down-conversion is an important process in quantum optics. A nonlinear crystal splits incoming photons into pairs of photons of lower energy whose combined energy and momentum are equal to the energy and momentum of the original photon. "Parametric" refers to the fact that the state of the crystal is left unchanged in the process, which is why energy and momentum must be conserved (this is related to phase matching in nonlinear optics). The process is "spontaneous" in the same sense as spontaneous emission—it is stimulated by random vacuum fluctuations. Consequently, the photon pairs are created at random times. However, if one of the pair (the "signal") is detected at any time then we know its partner (the "idler") is present. This then allows for the creation of optical fields containing a single photon. As of 2005, this is predominant mechanism for experimentalists to create single photons (also known as Fock states). The single photons as well as the photon pairs are often used in quantum information experiments and applications like quantum cryptography and the Bell test experiments.