Strong on-Chip Microwave Photon-Magnon Coupling Using Ultralow-Damping Epitaxial Y 3 Fe 5 O 12 Films at 2 K.

YFeO 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 YFeO thin films grown on a diamagnetic YScGaO 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 toward scalable hybrid quantum systems that integrate superconducting microwave resonators, YIG film magnon conduits, and superconducting qubits into on-chip QIS devices.