# Exceptional magnetic sensitivity of PT-symmetric cavity magnon polaritons

## TL;DR

This paper predicts that PT-symmetric cavity magnon polaritons can achieve ultrahigh magnetic sensitivity near exceptional points, surpassing current magnetometer performance, with potential experimental realization.

## Contribution

It introduces the concept of using PT symmetry in cavity magnon polaritons to enhance magnetic sensitivity at room temperature, a novel approach in non-Hermitian spintronics.

## Key findings

- Ultranarrow linewidth spectrum in the exact PT phase.
- Ultrahigh magnetic sensitivity approaching 10^{-15} T/Hz^{1/2}.
- Spectral evolution to a step function at the third-order exceptional point.

## Abstract

Achieving magnetometers with ultrahigh sensitivity at room temperature is an outstanding problem in physical sciences and engineering. Recently developed non-Hermitian cavity spintronics offers new possibilities. In this work we predict an exceptional magnetic sensitivity of cavity magnon polaritons with the peculiar parity-time (PT) symmetry. Based on the input-output formalism, we demonstrate a "Z"-shape spectrum including two sideband modes and a dark-state branch with an ultranarrow linewidth in the exact PT phase. The spectrum evolves to a step function when the polariton touches the third-order exceptional point, accompanied by an ultrahigh sensitivity with respect to the detuning. The estimated magnetic sensitivity can approach 10^{-15}T Hz^{-1/2} in the strong coupling region, which is two orders of magnitude higher than that of the state-of-the-art magnetoelectric sensor. We derive the condition for the noiseless sensing performance. Purcell-like effect is observed when the PT symmetry is broken. A possible experimental scheme to realize our proposal is also discussed.