Rigidity of an ordered phase in condensed matter results in collectiveexcitation modes spatially extending in macroscopic dimensions. Magnon is aquantum of an elementary excitation in the ordered spin system, such asferromagnet. Being low dissipative, dynamics of magnons in ferromagneticinsulators has been extensively studied and widely applied for decades in thecontexts of ferromagnetic resonance, and more recently of Bose-Einsteincondensation as well as spintronics. Moreover, towards hybrid systems forquantum memories and transducers, coupling of magnons and microwave photons ina resonator have been investigated. However, quantum-state manipulation at thesingle-magnon level has remained elusive because of the lack of anharmonicelement in the system. Here we demonstrate coherent coupling between a magnonexcitation in a millimetre-sized ferromagnetic sphere and a superconductingqubit, where the interaction is mediated by the virtual photon excitation in amicrowave cavity. We obtain the coupling strength far exceeding the dampingrates, thus bringing the hybrid system into the strong coupling regime.Furthermore, we find a tunable magnon-qubit coupling scheme utilising aparametric drive with a microwave. Our approach provides a versatile tool forquantum control and measurement of the magnon excitations and thus opens a newdiscipline of quantum magnonics.
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