**Measurement of Geometric phase for mixed states using single photon interferometry** - Marie Ericsson

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 mixed input states. I will talk about the first experiment to measure mixed state geometric phases in optics. The experiment is performed with a Mach-Zehnder interferometer, using polarization mixed state photons that are produced in two different ways: decohering a pure state single photon with a birefringent element; and producing a nonmaximally entangled state of two photons and tracing over one of them. The mixed state geometric phase data are in very good agreement with theoretical predictions.