Xiao Lei, Deng Tianshu, Wang Kunkun, Wang Zhong, Yi Wei, Xue Peng
Beijing Computational Science Research Center, Beijing 100084, China.
Institute for Advanced Study, Tsinghua University, Beijing 100084, China.
Phys Rev Lett. 2021 Jun 11;126(23):230402. doi: 10.1103/PhysRevLett.126.230402.
Parity-time (PT)-symmetric Hamiltonians have widespread significance in non-Hermitian physics. A PT-symmetric Hamiltonian can exhibit distinct phases with either real or complex eigenspectrum, while the transition points in between, the so-called exceptional points, give rise to a host of critical behaviors that holds great promise for applications. For spatially periodic non-Hermitian systems, PT symmetries are commonly characterized and observed in line with the Bloch band theory, with exceptional points dwelling in the Brillouin zone. Here, in nonunitary quantum walks of single photons, we uncover a novel family of exceptional points beyond this common wisdom. These "non-Bloch exceptional points" originate from the accumulation of bulk eigenstates near boundaries, known as the non-Hermitian skin effect, and inhabit a generalized Brillouin zone. Our finding opens the avenue toward a generalized PT-symmetry framework, and reveals the intriguing interplay between PT symmetry and non-Hermitian skin effect.
宇称-时间（PT）对称哈密顿量在非厄米物理中具有广泛的意义。一个PT对称哈密顿量可以表现出具有实或复本征谱的不同相，而其间的转变点，即所谓的例外点，会引发一系列极具应用前景的临界行为。对于空间周期性非厄米系统，PT对称性通常根据布洛赫能带理论来表征和观测，例外点位于布里渊区内。在此，在单光子的非幺正量子行走中，我们发现了超出这一常识的一类新型例外点。这些“非布洛赫例外点”源于边界附近体本征态的积累，即所谓的非厄米趋肤效应，并存在于一个广义布里渊区内。我们的发现开辟了通往广义PT对称框架的道路，并揭示了PT对称性与非厄米趋肤效应之间有趣的相互作用。
