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.