  We discuss different proposals for the degree of polarization of quantum
fields. The simplest approach, namely making a direct analogy with the
classical description via the Stokes operators, is known to produce
unsatisfactory results. Still, we argue that these operators and their
properties should be basic for any measure of polarization. We compare
alternative quantum degrees and put forth that they order various states
differently. This is to be expected, since, despite being rooted in the Stokes
operators, each of these measures only captures certain characteristics.
Therefore, it is likely that several quantum degrees of polarization will
coexist, each one having its specific domain of usefulness.

Quantum Physics