  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.
