When I search for "on demand photon pairs" I find that most publications are about entangled pairs. I found no papers and articles about on-demand pairs where the photons are identical in all respects except their direction, and whose polarization & frequency are the same across pairs and within pairs. It always seems to involve one other parameter (e.g. polarization) whose value is random from pair to pair, but is correlated across each pair. Just wondering whether this is because (A) entangled on-demand pairs are more useful and interesting than non-entangled ones OR (B) because there is some theoretical constraint that makes it necessary that they should have an additional degree of freedom (apart from direction) and that they should have correlated values in that degree of freedom.