As far as I know first entanglement experiment used polarization entangled photons from atomic cascade. As I understand atomic cascade produces entangled photons from two electrons that emit photons by falling from the same orbital to the same lower orbital (so they have the same spin). Now it seems that each photon from pair is entangled with it's electron. So can we consider two photons directly entangled? Naively it would seem more reasonable to view it differently: if we measure polarization of one photon it should "collapse" spin of the electron that emitted it. This electron in turn "collapses" the spin of other electron in the same orbital and that in turn "collapses" polarization state of other photon.