  Using a spontaneous parametric-downconversion source of photon pairs, we are
working towards the creation of arbitrary 2-qubit quantum states with high
fidelity. Currently, all physically allowable combinations of polarization
entanglement and mixture can be produced, including maximally-entangled mixed
states. The states are experimentally measured and refined via
computer-automated quantum-state tomography, and this system has also been used
to perform single-qubit and ancilla-assisted quantum process tomography.
