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 ($\mathcal{P}\mathcal{T}$) 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 $\mathcal{P}\mathcal{T}$ 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 ${10}^{-15}\mathrm{T}{\mathrm{Hz}}^{-1/2}$ 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 $\mathcal{P}\mathcal{T}$ symmetry is broken. A possible experimental scheme to realize our proposal is also discussed.

• Received 24 January 2019
• Revised 22 May 2019

DOI:https://doi.org/10.1103/PhysRevB.99.214415