  James Clerk Maxwell unknowingly discovered a correct relativistic, quantum
theory for the light quantum, forty-three years before Einstein postulated the
photon's existence. In this theory, the usual Maxwell field is the quantum wave
function for a single photon. When the non-operator Maxwell field of a single
photon is second quantized, the standard Dirac theory of quantum optics is
obtained. Recently, quantum-state tomography has been applied to experimentally
determine photon wave functions.
