Non-Hermitian Quantum Mechanics in Photonic Systems: From Fundamental Principles to Practical Devices

Non-Hermitian quantum mechanics has emerged as a transformative framework in modern photonics, challenging the conventional Hermitian paradigm that has long governed quantum systems. By relaxing the requirement of Hermiticity, novel physical concepts such as parity-time (PT) symmetry, exceptional points, and gain–loss engineering become accessible, offering fundamentally new pathways to manipulate light–matter interactions. In recent years, these ideas have evolved from theoretical constructs into experimentally realizable phenomena across diverse platforms, including photonic crystals, coupled waveguides, microring resonators, and metasurfaces. This review provides a comprehensive overview of the theoretical foundations of non-Hermitian quantum mechanics, its experimental realizations in photonic systems, and its practical applications. Particular emphasis is placed on exceptional-point physics, ultra-sensitive sensors, non-Hermitian lasers, coherent perfect absorbers, and topological photonics. We further discuss the key challenges in material design, device scalability, and noise robustness that currently limit practical deployment, alongside emerging research directions that hold promise for integration into next-generation quantum technologies. By synthesizing progress across theory, experiment, and application, this review highlights the significance of non-Hermitian approaches in shaping the future of photonic science and technology.