 which is invariant under rotation of the \(x\) and \(y\) axes in the plane perpendicular to \(z\). The total quantum state of the pair of photons 1 and 2 is invariant under the exchange of the two photons, as required by the fact that photons are integral spin particles. Suppose now that photons 1 and 2 impinge respectively on the faces of birefringent crystal polarization analyzers I and II, with the entrance face of each analyzer perpendicular to \(z\). Each analyzer has the property of separating light incident upon its face into two outgoing non-parallel rays, the ordinary ray and the extraordinary ray. The transmission axis of the analyzer is a direction with the property that a photon polarized along it will emerge in the ordinary ray (with certainty if the crystals are assumed to be ideal), while a photon polarized in a direction perpendicular to \(z\) and to the transmission axis will emerge in the extraordinary ray. See Figure 1: