Photon wave function: A covariant formulation and equivalence with QED

We discuss the limits of the photon wave function (PWF) formalism, which is experiencing a revival these days as a result of new practical applications in photonics and quantum optics. We build a Dirac-like equation for the PWF written in a manifestly covariant form and show that, in the presence of charged matter fields, it reproduces the standard formulation of (classical) electrodynamics. This shows that attempts to construct a full quantum theory of interacting photons (mutually or interacting with matter) based on the so-called photon wave function approach can lead only to results already described by standard quantum electrodynamics (QED). The PWF formalism can then be used only to provide an easier description of some particular situations—for example, the propagation of free photons or photons propagating in a medium as described in Bialynicki-Birula [in Progress in Optics, edited by E. Wolf (Elsevier, Amsterdam, 1996), pp. 245–294] especially when the photon number remains fixed in time but not to replace QED in toto.