Critical behaviour of coherence and correlation of counterpropagating twin beams

This work analyses the temporal coherence and correlation of twin beams generated in a quasi-phase matched nonlinear crystal in a counterpropagating configuration, ranging from the low-gain regime, where counterpropagating photon pairs are generated spontaneously, to the regime of stimulated pair production, close to the MOPO (Mirrorless Optical Parametric Oscillator) threshold. Here we show a critical divergence of the correlation time and slowing down of quantum fluctuations originating from the feedback mechanism responsible of the MOPO threshold.