Preparation of pure and mixed polarization qubits and the direct   measurement of figures of merit
  Non-classical joint measurements can hugely improve the efficiency with which certain figures of merit of quantum systems are measured. We use such a measurement to determine a particular figure of merit, the purity, for a polarization qubit. In the process we highlight some of subtleties involved in common methods for generating decoherence in quantum optics. 
R. B. A. Adamson, L. K. Shalm, A. M. Steinberg