Simultaneous realization of nonreciprocal and ultra-strong coupling in cavity magnonics

Cavity magnonics, a hybrid platform bridging photonics and spintronics, faces a fundamental challenge in reconciling ultra-strong light-matter interactions with nonreciprocal signal control. By introducing a yttrium iron garnet defect into a photonic crystal, we demonstrate ultra-strong coupling with a coupling strength of 1.18 GHz and 10.9% energy transfer efficiency at room temperature. Time-reversal symmetry breaking via gyromagnetic and Faraday effects induces nonreciprocal microwave transmission within the photonic bandgap, achieving 20 dB isolation over a 1.2 GHz bandwidth. Crucially, the emergent isolation functionality originates from fundamental magnon-photon coupling control rather than engineered device optimization, providing a phenomenological framework for coherent signal manipulation in hybrid quantum systems. Our simultaneous realization of ultra-strong and nonreciprocal coupling at room temperature establishes a new paradigm for scalable quantum transducers and reconfigurable microwave isolators, overcoming the cryogenic limitations of superconducting systems.