Synthesizing arbitrary two-photon polarization mixed states
  Two methods for creating arbitrary two-photon polarization pure states are introduced. Based on these, four schemes for creating two-photon polarization mixed states are proposed and analyzed. The first two schemes can synthesize completely arbitrary two-qubit mixed states, i.e., control all 15 free parameters: Scheme I requires several sets of crystals, while Scheme II requires only a single set, but relies on decohering the pump beam. Additionally, we describe two further schemes which are much easier to implement. Although the total capability of these is still being studied, we show that they can synthesize all two-qubit Werner states, maximally entangled mixed states, Collins-Gisin states, and arbitrary Bell-diagonal states. 
Tzu-Chieh Wei (1), Joseph B. Altepeter (1), David Branning (1,2), Paul   M. Goldbart (1), D. F. V. James (3), Evan Jeffrey (...