Enhancing spin-photon coupling with a micromagnet

Hybrid quantum systems involving solid-state spins and superconducting microwave cavities play a crucial role in quantum science and technology, but improving the spin-photon coupling at the single quantum level remains challenging in such systems. Here, we propose a simple technique to strongly couple a single solid-state spin to the microwave photons in a superconducting coplanar waveguide cavity via a magnetic microsphere. We show that strong coupling at the single spin level can be realized by virtual magnonic excitations of a nearby micromagnet. The spin-photon coupling strength can be enhanced up to typically 4 orders of magnitude larger than that without the use of the micromagnet. This work can find applications in quantum information processing with strongly coupled solid-state spin-photonic systems.