Observation of Non-Bloch Parity-Time Symmetry and Exceptional Points

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.

Automated summary

PT-symmetric Hamiltonians play a significant role in non-Hermitian physics, exhibiting distinct phases and exceptional points. In the nonunitary quantum walks of single photons, a novel family of exceptional points beyond the common Bloch theory have been discovered, originating from the non-Bloch exceptional points and showing an intriguing connection with the non-Hermitian skin effect.