Exceptional magnetic sensitivity of P T -symmetric cavity magnon polaritons

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 (P T ) symmetry. Based on the input-output formalism, we demonstrate a "Z"-shape spectrum including two side-band modes and a dark-state branch with an ultranarrow linewidth in the exact P T 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^-15T 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 P T symmetry is broken. A possible experimental scheme to realize our proposal is also discussed.