2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/92622Geometric 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.To appear in Phys. Rev. Lett. 4 pages, 3 figuresQuantum Physicstext