Observation of topological phenomena in a Weyl exceptional ring with single photons

Compared with Hermitian theory, non-Hermitian physics offers a fundamentally different mathematical framework, enabling the observation of topological phenomena that have no analogue in Hermitian systems. Among these, the exceptional ring - a ring of exceptional points (EP) - stands out as a quintessential topological feature unique to non-Hermitian systems. In this study, we employ single-photon interferometry to overcome the experimental challenge of precise phase control in quantum systems, thereby enabling a full simulation of the non-Hermitian exceptional ring in the three-dimensional parameter space, while also validating, at the experimental level, the symmetry assumptions required for the measurement. Moreover, we explicitly demonstrate the coherent dynamical evolution of the system, which constitutes a clear manifestation of its quantum nature. By measuring the dynamics in parameter space and mapping them onto reciprocal space, we determine the system's eigenstates, which allows us to characterize the topological band structure of the system under different conditions. We describe the topological properties of the exceptional ring by extracting the Chern number and Berry phase for different parameter manifolds and observe the topological critical phenomena of the system. Moreover, our experimental approach can be extended to probe higher-order EP topologies. Our work paves the way for further exploration of topological non-Hermitian systems.