Non-Hermitian physics in photonic systems

Abstract

Non-Hermitian physics, which studies systems with nonconservative interactions with the environment, has attracted growing interest. Unique phenomena such as enhanced sensing, the non-Hermitian skin effect, and novel topological behaviors have been extensively explored. Photonic systems serve as ideal platforms to investigate these effects because of inherent decoherence and loss of photons. Non-Hermitian photonics not only reveals intriguing physical effects but also offers new avenues for manipulating light and its associated information, enabling device-level applications. This review summarizes recent progress in non-Hermitian photonic systems. We begin by introducing key concepts and theorems, including the connection between master equations and non-Hermitian Hamiltonians, pseudo-Hermiticity, parity-time symmetry, conserved quantities, exceptional points, and biorthogonal theory. Based on this foundation, we discuss how various platforms - including bulk optics, waveguides, optical cavities, fibers, synthetic dimensions, and metamaterials - simulate non-Hermitian systems. Particular focus is given to the construction of effective non-Hermitian Hamiltonians and operators. We introduce the non-Bloch band theory through a photonic quantum walk platform, highlighting the roles of non-Hermitian topology under complex spectra, which may stimulate advances in both fundamental research and practical applications. Finally, we review unique phenomena and potential applications in sensing, chiral state transfer, quantum algorithms, and other emerging non-Hermitian photonic devices.