Temporal coherence and correlation of counterpropagating twin 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.