Temporal coherence and correlation of coounter-propagating twin photons

Among the few sources of quantum entangled light, parametric down-conversion in ?(2) media is one of the most accessible and widely employed. Here we focus on a non-conventional configuration, where twin photons are generated in opposite directions thanks to quasi phase-matching in periodically poled crystal. Although predicted almost 50 years ago, counter-propagating three-wave interactions have been only recently realized [1], thanks to technical advancements in achieving the required sub-micrometer poling periods.