Non-Reciprocal Exceptional Points in Magnon-Photon Systems: Theory, Experiment, and Outlook

Exceptional points (EPs) in non-Hermitian systems have been proposed to enhance sensing, but their effectiveness remains debated. In this work, we realize a tunable, non-Hermitian architecture by coupling a Yttrium Iron Garnet (YIG) sphere to a microwave cavity. We demonstrate amplitude- and phase-tunable interactions between the cavity and YIG using unidirectional amplifiers, digital attenuators, and phase shifters. In contrast to PT-symmetric EPs, we study non-reciprocal EPs traversed by the detuning introduced by external magnetic fields. We observe that the system's response to external magnetic fields around the EPs is heightened, showcasing the square root response characteristic of EPs. However, practical challenges, such as parameter hypersensitivity and reduced dynamic range, limit the potential for robust sensor applications using a linear EP approach. Despite these limitations, we observe promising effects from certain nonlinear phenomena that may help overcome these barriers. This work outlines the critical characteristics of non-reciprocal EP-based sensing and provides an outlook for potential nonlinear enhancements.