On the paraxial approximation in quantum optics II: Henochromatic modes of a Maxwell field

A companion paper has argued that the best way to associate single-particle quantum states of a scalar field to the modes of a narrowly collimated beam of classical radiation modeled in the paraxial approximation uses the ``henochromatic’’ states previously introduced by Sudarshan, Simon and Mukunda. This paper extends that result to Maxwell fields, again emphasizing the central role of unitarity in defining the association. The principal new technical element in the present discussion has to do with the intertwining of polarization and spatial degrees of freedom in the resulting single-photon states.