Enhancement of the coupling between a microwave photon and a color-center spin transition via a magnon mode*

Hybrid quantum systems are attractive for emerging quantum technologies because they take advantage of the distinct properties of the constituent excitations; chiefly at the ultrastrong coupling regime, where the relaxation rates of the distinct quantum systems are exceeded by their coupling rate. Thus, a central challenge is to couple distinct quantum systems strongly, with all elements maintaining long coherence times. Magnetic materials are promising for coupling to long-lived quantum spin systems, potentially enabling quantum interconnects between important quantum technologies based on microwave photons and color-center spins (CCS). We describe an approach that dramatically enhances the effective coupling between microwave photons and CCS. Near the surface of a magnet, the color-center spins feel a strong dipole-dipole interaction with the nearby lattice of spin-polarized ions in the magnet. We predict that this microwave-magnon-spin coupling leads to an avoided crossing between the microwave photon and the CCS that dramatically exceeds the direct coupling between them.