  Quantum descriptions of polarization show the rich degrees of freedom
underlying classical light. While changes in polarization of light are
well-described classically, a full quantum description of polarimetry, which
characterizes substances by their effects on incident light's polarization, is
lacking. We provide sets of quantum channels that underlie classical
polarimetry and thus correspond to arbitrary Mueller matrices. This allows us
to inspect how the quantum properties of light change in a classical
polarimetry experiment, and to investigate the extra flexibility that quantum
states have during such transformations. Moreover, our quantum channels imply a
new method for discriminating between depolarizing and nondepolarizing Mueller
matrices, which has been the subject of much research. This theory can now be
taken advantage of to improve estimation strategies in classical polarimetry
and to further explore the boundaries between classical and quantum
polarization.
