Abstract
Achieving extraordinarily high sensitivity is a long-sought goal in the development of novel and more capable electromagnetic sensors. We present here how a coherent perfect absorber-laser (CPAL) enabled by parity-time (PT) symmetry breaking may be exploited to build ultrasensitive monochromatic electromagnetic sensors that use radio waves, microwaves, terahertz radiations, or light. We argue the possibility of using such CPAL sensors to detect extremely small-scale perturbations of admittance or refractive index caused by, for example, low-density gas molecules and microscopic properties, as they may drastically vary the system's output intensity from very low (coherent absorption) to high (lasing). We derive the physical bounds on CPAL sensors, showing that their sensitivity and resolvability may go well beyond traditional electromagnetic sensors, such as sensors based on Fabry-Perot cavities.