Strong spin-magnon coupling in a van der Waals magnet with tunable chiral symmetry.

Photons have long been the dominant mediators of interactions in cavity quantum electrodynamics (QED), with recent advances in chiral cavities enabling directional light-matter coupling. Extending these capabilities to spin qubits, however, remains a major challenge, motivating the exploration of alternative solid-state platforms. Here, we report the experimental demonstration of a chiral magnonic cavity, where magnons-collective excitations of magnetically ordered solids-replace photons. We demonstrate strong spin-magnon coupling between [Gd(W5O18)2]9- spin qubits and antiferromagnetic magnons in the van der Waals material CrSBr, observing clear anticrossings and coherent hybridization. Crucially, we exploit the intrinsic chirality of antiferromagnetic magnons to tune the coupling strength by simply rotating an external magnetic field, dynamically controlling the magnon symmetry from linear to chiral within a single device. Our results establish magnon QED as a powerful platform for spin qubit research and introduce chirality as a control knob for information flow.