  We show that a configuration of four birefringent crystals and wave-plates
can emulate almost any arbitrary unital channel for polarization qubits encoded
in single photons, where the channel settings are controlled by the wave-plate
angles. The scheme is applied to a single spatial mode and its operation is
independent of the wavelength and the fine temporal properties of the input
light. We implemented the scheme and demonstrated its operation by applying a
dephasing environment to classical and quantum single-photon states with
different temporal properties. The applied process was characterized by a
quantum process tomography procedure, and a high fidelity to the theory was
observed.
