Cavity Enhanced Spontaneous Parametric Down-Conversion for the Prolongation of Correlation Time between Conjugate Photons

By resonating both fields of the spontaneous parametric down-conversion with an optical cavity, the bandwidth of the fields is reduced to a level that can be resolved by photodetectors. This enables us to directly measure the distribution of the time intervals between the two down-converted photons. The correlation time is found inversely proportional to the down-conversion bandwidth. In the meantime, the signal levels of the fields are greatly enhanced by the resonance. Such a two-photon source will find wide application in quantum information processing.