Controllable generation of mixed two-photon states

We report a controllable method for producing mixed two-photon states via spontaneous parametric down-conversion with a two-type-I crystal geometry. By using variable polarization rotators (VPRs), one obtains mixed states of various purities and degrees of entanglement depending on the parameters of the VPRs. The generated states are characterized by quantum state tomography. The experimental results are found to be in good agreement with the theory. The method can be easily implemented for various experiments that require the generation of states with controllable degrees of entanglement or mixedness. (paper)