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

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.