  We present a theoretical approach to use ferro- or ferrimagnetic
nanoparticles as microwave nanomagnonic cavities to concentrate microwave
magnetic fields into deeply subwavelength volumes $\sim 10^{-13}$ mm$^3$. We
show that the field in such nanocavities can efficiently couple to isolated
spin emitters (spin qubits) positioned close to the nanoparticle surface
reaching the single magnon-spin strong-coupling regime and mediate efficient
long-range quantum state transfer between isolated spin emitters. Nanomagnonic
cavities thus pave the way towards magnon-based quantum networks and
magnon-mediated quantum gates.
