Quantum process tomography of single photon quantum channels with   controllable decoherence
  We characterized unital quantum channels of single photon polarization qubits. The channels are composed of two birefringent crystals and wave-plates, where their decoherence properties are controlled. An experimental comparison between two different depolarizing configurations was performed using a quantum process tomography procedure. The results are with a high fidelity to the theoretical predictions. 
A. Shaham and H. S. Eisenberg
quant-ph