  Rigidity of an ordered phase in condensed matter results in collective
excitation modes spatially extending in macroscopic dimensions. Magnon is a
quantum of an elementary excitation in the ordered spin system, such as
ferromagnet. Being low dissipative, dynamics of magnons in ferromagnetic
insulators has been extensively studied and widely applied for decades in the
contexts of ferromagnetic resonance, and more recently of Bose-Einstein
condensation as well as spintronics. Moreover, towards hybrid systems for
quantum memories and transducers, coupling of magnons and microwave photons in
a resonator have been investigated. However, quantum-state manipulation at the
single-magnon level has remained elusive because of the lack of anharmonic
element in the system. Here we demonstrate coherent coupling between a magnon
excitation in a millimetre-sized ferromagnetic sphere and a superconducting
qubit, where the interaction is mediated by the virtual photon excitation in a
microwave cavity. We obtain the coupling strength far exceeding the damping
rates, thus bringing the hybrid system into the strong coupling regime.
Furthermore, we find a tunable magnon-qubit coupling scheme utilising a
parametric drive with a microwave. Our approach provides a versatile tool for
quantum control and measurement of the magnon excitations and thus opens a new
discipline of quantum magnonics.
