  Depolarization is one of the most important sources of error in a quantum
communication link that can be introduced by the quantum channel. Even though
standard quantum process tomography can, in theory, be applied to characterize
this effect, in most real-world implementations depolarization cannot be
distinguished from time-varying unitary transformations, especially when the
time scales are much shorter than the detectors response time. In this paper,
we introduce a method for distinguishing true depolarization from fast
polarization rotations by employing Hong-Ou-Mandel interference. It is shown
that the results are independent of the timing resolutions of the
photodetectors.
