Optical parametric emission produces light in which photons are created in highly correlated pairs. This feature is formalized through the concept of quadratic coherence. We study the correlation between twin photons through a spectral and temporal analysis of the second-harmonic of the parametric light.
Temporally incoherent twin beams of light are generated by means of an optical parametric amplifier that is pumped by the second harmonic of a Q-switched multi-longitudinal-mode Nd:YAG laser. For a parametric gain of 1·12, the intensity noise levels on the direct-detected signal and idler twin beams are found to be correlated by 1·4 dB, whereas the subtracted noise falls below the quantum limit by 0·7 ± 0·1 dB (15%). Although no direct coherence experiment is performed on the twin beams themselves, their temporal incoherence is manifested by the stringent path-length matching requirement between the pump and input signal beams.
In this paper we discuss a new, two-mode formalism1 for analyzing two-photon optical devices (e.g., parametric amplifiers and four-wave mixers)–devices whose output light is generated by the simultaneous emission of two photons into two of the output modes. Such devices can produce in principle squeezed-state light, in which the two modes of each pair of output modes have correlated noise. Two-photon devices are to be contrasted with one-photon devices, such as the laser, whose output light is generated by the emission of independent single photons.
This work analyzes the temporal coherence and correlation of counterpropagating twin photons generated in a quasiphase matched nonlinear crystal by spontaneous parametric down-conversion. We find out different pictures depending on the pump pulse duration relative to two characteristic temporal scales, determined, respectively, by the temporal separation between the counterpropagating and the co-propagating wave packets. When the pump duration is intermediate between the two scales, we show a transition from a highly entangled state to an almost separable state, with strongly asymmetric spectral properties of the photons.
An approach for calculating fourth-order coherence functions between fields produced in the parametric down-conversion process is proposed. It takes into account the transverse correlations between twin photons and is used to calculate the visibility of the interference patterns obtained in double-slit experiments with coincidence detection. The finite dimension of the light source and detectors are shown to be responsible for a kind of partial coherence which affects the visibility of the fringes. The results explain known features and predict other ones.