 Photon pairs are emitted from the source, each pair quantum mechanically described by \(\ket{\Phi}\) of Eq. (15). I and II are polarization analyzers, with outcome \(s=1\) and \(t=1\) designating emergence in the ordinary ray, while \(s = -1\) and \(t = -1\) designate emergence in the extraordinary ray. The crystals are also idealized by assuming that no incident photon is absorbed, but each emerges in either the ordinary or the extraordinary ray. 