  Y3Fe5O12 is arguably the best magnetic material for magnonic quantum
information science (QIS) because of its extremely low damping. We report
ultralow damping at 2 K in epitaxial Y3Fe5O12 thin films grown on a diamagnetic
Y3Sc2Ga3O12 substrate that contains no rare-earth elements. Using these
ultralow damping YIG films, we demonstrate for the first time strong coupling
between magnons in patterned YIG thin films and microwave photons in a
superconducting Nb resonator. This result paves the road towards scalable
hybrid quantum systems that integrate superconducting microwave resonators, YIG
film magnon conduits, and superconducting qubits into on-chip QIS devices.
