The Dirac notation, though originally applied to the propagation of single particles, also applies to describing the propagation of ensembles of cohe… - F. J. Duarte

The two photons are entangled and according to local realism, their polarization planes should become independent... a typical EPR situation. Already in 1948, observations... agreed with quantum mechanics, not with local realism.

The intimate relation between interference and diffraction has its origin in the interference equation itself.

Regardless of the prophetic value of Dirac’s description [on interference] his was probably the first discussion... including a coherent beam of light. In other words, Dirac wrote the first chapter in laser optics.