Basic Process
A nonlinear crystal is used to split photons into pairs of photons that, in accordance with the law of conservation of energy, have combined energies and momenta equal to the energy and momentum of the original photon, are phase-matched in the frequency domain, and have correlated polarizations. (The state of the crystal is unchanged by the process.) If the photons share the same polarization it is deemed Type I correlation; if they have perpendicular polarisations it is deemed Type II. The fundamental reason why some of the photons are split is not understood.
SPDC is stimulated by random vacuum fluctuations, and hence the photon pairs are created at random times. The conversion efficiency is very low, on the order of 1 pair per every 10^12 incoming photons. However, if one of the pair (the "signal") is detected at any time then its partner (the "idler") is known to be present. The output of a Type I down converter is a squeezed vacuum that contains only even photon number terms. The output of the type II down converter is a two-mode squeezed vacuum.
Read more about this topic: Spontaneous Parametric Down-conversion
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