Enhanced sensing of anharmonic perturbations in a dissipatively coupled anti-PT symmetric system.*

Recently, non-Hermitian degeneracies known as exceptional points have unravelled a new direction to administer enhanced response in open quantum systems. However, most research activity based on exceptional points requires parity-time (PT) symmetry with a balanced gain-loss profile, and they are tailor-made to sense linear perturbations. But, physical systems are inherently anharmonic, and nonlinear models most accurately describe macroscopic physics.

Here, we demonstrate a new protocol for the enhanced sensing of linear and nonlinear perturbations based on a dissipatively coupled two-mode system with anti-PT symmetry. The sensitivity stems from the coherence between the modes induced by a shared reservoir. Owing to the dissipative coupling between the modes, the linear response acquires a pole on the real axis. We demonstrate how this singularity can be exploited for the enhanced sensing of weak perturbations at low pump powers. Our model is generic, relevant to a large class of physical systems, and the mechanism is specifically illustrated in the context of a cavity interacting dissipatively with a ferromagnet.