  The properties of coherence and polarization of light has been the subject of
intense investigations and form the basis of many technological applications.
These concepts which historically have been treated independently can now be
formulated under a single classical theory. Here, we derive a quantum
counterpart for this theory, with basis on a density matrix formulation, which
describes jointly the coherence and polarization properties of an ensemble of
photons. The method is used to show how the degree of polarization of a
specific class of mixed states changes on propagation in free space, and how an
interacting environment can suppress the coherence and polarization degrees of
a general state. This last application can be particularly useful in the
analysis of decoherence effects in optical quantum information implementations.
