Enhanced sensing of anharmonicities in a gain-based anti-PT symmetric system

We study the enhanced sensing of weak anharmonicities in a gain-based cavity-magnon-waveguide coupled system. By dissipatively coupling the two subsystems through a mediating waveguide, the Hamiltonian of the system is tailored to be anti-parity-time symmetric. Unique to the gain condition, the eigenvalues exhibit two singularities with linewidth suppression, distinguishing them from those of gain-free systems. Under the gain condition, a counter-intuitive bistable signature emerges even at low drive powers. As the effective gain approaches a certain value, this bistability yields a significantly enhanced spin-current response of the magnon mode. Consequently, the sensitivity, quantified by an enhancement factor, is enhanced remarkably compared to the linewidth suppression scenario. Moreover, the high enhancement factor can be sustained over a broad gain-bandwidth and also stays large even when the coherent coupling becomes considerably strong. Based on the integrated cavity-magnon-waveguide systems, this scheme can be used for sensing different physical quantities related to the Kerr-type nonlinearity and has potential applications in high-precision measuring microwave-signal nonlinearities.