  Controlling the depolarization of light is a long-standing open problem. In
recent years, many demonstrations have used the polarization of single photons
to encode quantum information. The depolarization of these photons is
equivalent to the decoherence of the quantum information they encode. We
present schemes for building various depolarizing channels with controlled
properties using birefringent crystals. Three such schemes are demonstrated and
their effects on single photons are shown by quantum process tomography to be
in good agreement with a theoretical model.
