Observing mixed-state Uhlmann winding numbers in integrated photonic quantum walks

arXiv:2611.07600v1 Announce Type: new Abstract: Integer winding numbers provide a well-established classification of topological phases protected by chiral symmetry at zero temperature, but extending this classification to thermal mixed states remains challenging. The Uhlmann phase, a mixed-state extension of the Berry phase, provides a geometric framework for characterizing finite-temperature topology. However, this geometric phase is restricted to two quantized values and therefore cannot resolve the multiple thermal regimes associated with high winding numbers. Here, we experimentally observe integer mixed-state winding numbers in a photonic quantum-walk chip.