Noise is a central property that characterizes quantum information channels. It is the result of an interaction between a system of interest and another inaccessible system, usually called the ‘environment’. In this work, we studied physical realizations of noisy quantum channels in a controllable manner, where the quantum information is encoded in the polarization of single photons. By using the coupling of time delay to the polarization in birefringent crystals, we implemented channels with different degrees of depolarization. A sequence of two birefringent crystals with a tunable relative angle and wave-plates, is enough to demonstrate different types of depolarization. Among the channels that we have demonstrated are a non-depolarizing channel, a partially anisotropic depolarizing channel, a completely dephasing channel and a completely depolarizing channel. The small number of degrees of freedom which is involved in this realization makes it easier to analyze theoretically the accurate action of these depolarizing configurations. Complete quantum process tomography has been used to characterize these channels, and results are in a good agreement with experiments. We have also found and demonstrated possibilities to simulate isotropic depolarization channels.

The demonstrated depolarizing channels are not limited to qubits. They are currently used to study the depolarization of biphotons, whose polarization state forms three- and four-dimensional spaces.