Cavity-enhanced parametric down-conversion as a source of correlated photons

The most common way of generating correlated photons is by parametric down-conversion in a nonlinear crystal in free space. However, a drawback of this technique is the extremely low photon flux available. We present a simple analysis which shows that cavity-enhanced parametric downconversion, using an arrangement similar to an optical parametric oscillator and operated well below threshold, can yield a substantially higher photon flux without significantly degrading the correlation between photons.