Sensing Applications of PT‐Symmetry in Non‐Hermitian Photonic Systems

Abstract In recent years, rapid advances in non‐Hermitian physics and PT‐symmetry have brought new opportunities for ultra‐sensitive sensing. Especially the presence of controllable non‐conservative processes in optical and photonic systems has triggered the development of singularity‐based sensing. By flexibly tuning gain, loss, and coupling strength, a series of high‐resolution sensing approaches can be realized, with the potential of on‐chip integration. Another important non‐Hermitian singularity is the coherent perfect absorption‐lasing (CPAL) point in the PT‐broken phase, which manifests the coexistence of lasing and CPA, exhibiting intriguing properties with considerable sensing potential. As a crucial method for quantum sensing and metrology, the interaction between light and alkali‐metal atomic ensembles promises unprecedented sensitivity in the measurement of ultra‐weak magnetic field, inertia, and time. Therefore, extending the study of PT‐symmetry and singularity‐based sensing from conventional solid‐state wave systems to diffusive systems such as atomic ensembles is attracting wide attention. In this review, the development of singularity‐based sensing in PT/anti‐PT symmetric non‐Hermitian systems is summarized, with a special focus on photonic platforms including integration with waveguides, microcavities, metasurface, etc. In addition, sensing applications with discussion further extended to atomic ensembles, projecting future research trends in the field.