Observation of non-Bloch parity-time symmetry and exceptional points

TL;DR

This paper reports the discovery of non-Bloch exceptional points in nonunitary quantum walks, revealing a new class of PT-symmetry phenomena linked to the non-Hermitian skin effect and expanding the understanding of non-Hermitian physics.

Contribution

It introduces the concept of non-Bloch exceptional points arising from the non-Hermitian skin effect, extending PT-symmetry analysis beyond traditional Bloch band theory.

Findings

Identification of non-Bloch exceptional points in quantum walks

Demonstration of the interplay between PT symmetry and the non-Hermitian skin effect

Expansion of PT-symmetry framework to generalized Brillouin zones

Abstract

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.