Measurement of geometric phase for mixed states using single photon   interferometry
  Geometric phase may enable inherently fault-tolerant quantum computation. However, due to potential decoherence effects, it is important to understand how such phases arise for {\it mixed} input states. We report the first experiment to measure mixed-state geometric phases in optics, using a Mach-Zehnder interferometer, and polarization mixed states that are produced in two different ways: decohering pure states with birefringent elements; and producing a nonmaximally entangled state of two photons and tracing over one of them, a form of remote state preparation. 
Marie Ericsson, Daryl Achilles, Julio T. Barreiro, David Branning,   Nicholas A. Peters, and Paul G. Kwiat