Abstract

In this study, we design a new magnetic field measurement model. Specifically, we use a single two-level atom coupled to two cavities to construct a parity–time-symmetric system supporting a third-order exceptional point. If a perturbation is applied to the atomic transition frequency, the eigenvalue of the system will change, and the eigenvalue change is proportional to the cubic root of the perturbation. If the perturbation comes from the magnetic field, a sensitive magnetic field measurement device is formed. By introducing gain and loss via the input–output field, we realize a third-order EP in the non-Hermitian Hamiltonian. Our analysis shows that the system exhibits a nonlinear response to magnetic field perturbations, leading to enhanced spectral sensitivity compared to conventional linear detectors. This equivalent EP based on a cavity-QED sensing scheme breaks the limitation of passive EP sensors and provides a new theoretical idea for the design of sensitive magnetic field measurement devices.

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