Spontaneous parametric down-conversion

Spontaneous parametric down-conversion (SPDC) 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. 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). This phase matching dictates that the photon pair be entangled in the frequency domain. SPDC is stimulated by random vacuum fluctuations, as such 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. Either signal and idler photons share the same polarisation (type I) or they have perpendicular polarisations (type II). The output of a type I down converter is a squeezed vacuum, which contains only even photon number terms. The output of the type II down converter is a two-mode squeezed vacuum.

SPDC allows for the creation of optical fields containing (to a good approximation) a single photon. As of 2005, this is the 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.