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Observation of Non-Hermitian Spectral Deformation in Complex Momentum Space
Mu Yang1, Yue Li1, Mingtao Xu1
1University of Science and Technology of China, University of Science and Technology of China, University of Science and Technology of China, University of Science and Technology of China, Laboratory of Quantum Information, Hefei 230026, China, Anhui Province Key Laboratory of Quantum Network, Hefei, Anhui 230026, China, CAS Center For Excellence in Quantum Information and Quantum Physics, Hefei 230026, China, and Hefei National Laboratory, Hefei 230088, China.
Abstract:
Open systems feature a variety of phenomena that arise from non-Hermitian physics. Recent theoretical studies have offered many insights into these phenomena through the non-Bloch band theory, though many of the theory's key features are experimentally elusive. In particular, the correspondence between complex momenta and non-Hermitian bands, while central to non-Bloch band theory, has so far defied direct experimental observation. Here we experimentally study the non-Hermitian spectral deformation in complex-momentum space, by implementing a non-Hermitian lattice with long-range couplings in the synthetic orbital-angular-momentum (OAM) dimension of photons inside a degenerate cavity. Encoding the complex momenta in the phase and amplitude modulations of the OAM modes, and devising a complex-momentum-resolved projective detection, we reconstruct the spectral deformation in complex-momentum space, where the eigenspectrum on the complex plane morphs through distinct geometries. This enables us to experimentally extract key information of the system under the non-Bloch band theory, including exceptional points in the complex-momentum space, the open-boundary spectra, and the generalized Brillouin zone. Our work demonstrates a versatile platform for exploring non-Hermitian physics and non-Bloch band theory, and opens the avenue for direct experimental investigation of non-Bloch features in the complex-momentum space.
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