Observation of Non-Hermitian Bulk-Boundary Correspondence in Nonchiral Nonunitary Quantum Dynamics of Single Photons

Non-Hermitian bulk-boundary correspondence, a central principle in non-Hermitian topological physics, is deeply rooted in symmetry. In particular, preserved chiral symmetry is one of the key ingredients, which plays a pivotal role in determining non-Hermitian topology. Nevertheless, chiral symmetry breaking in non-Hermitian systems substantially reshapes spectral and edge-state behavior. The corresponding fundamentally important bulk-boundary correspondence thus needs to be drastically reconstructed. However, it has so far eluded experimental efforts. Here, we theoretically predict and experimentally demonstrate the bulk-boundary correspondence of a one-dimensional (1D) non-Hermitian system with chiral symmetry breaking in discrete-time nonchiral nonunitary quantum walks of single photons. Through constructing a domain-wall configuration, we experimentally observe the photon localization at the interface of a domain-wall structure, clearly indicating the presence of the topological edge mode. The appearance of that matches excellently with the prediction of our newly introduced nonchiral non-Bloch topological invariants pair. Our Letter thus builds the non-Hermitian bulk-boundary correspondence for studying topological physics in non-chiral non-Hermitian systems.