Observation of Lossless Topological Bound States from Non-Hermitian Subspaces

Topological phases of matter, with their quantized invariants, offer the potential for disorder-resistant transport via topological bound states. Contrary to the belief that dissipation disrupts Hermiticity and Zak phase quantization, theoretical and experimental evidence of their persistence in a non-Hermitian photonic waveguide array is presented. A three-layer Su-Schrieffer-Heeger (SSH) chain is demonstrated, which can be split into a Hermitian SSH subspace and a non-Hermitian ladder subspace through hidden symmetry. This division allows the SSH subspace to retain its topological properties, resulting in a quantized Zak phase and lossless topological bound states. Additionally, the non-Hermitian subspace supports coherent transport dynamics, with the phase and intensity of bound states fixed at two extreme SSH layers, confirming the presence of the Hermitian subspace. These findings enhance the understanding of the interplay between non-Hermiticity and topology and pave the way for coherent topological light transport.