title: Hybridizing ferromagnetic magnons and microwave photons in the quantum limit.
abstract: We demonstrate large normal-mode splitting between a magnetostatic mode (the Kittel mode) in a ferromagnetic sphere of yttrium iron garnet and a microwave cavity mode. Strong coupling is achieved in the quantum regime where the average number of thermally or externally excited magnons and photons is less than one. We also confirm that the coupling strength is proportional to the square root of the number of spins. A nonmonotonic temperature dependence of the Kittel-mode linewidth is observed below 1 K and is attributed to the dissipation due to the coupling with a bath of two-level systems.
authors.affiliation: Research Center for Advanced Science and Technology (RCAST), The University of Tokyo, Meguro-ku, Tokyo 153-8904, Japan.
authors.forename: Yutaka
authors.initials: Y
authors.lastname: Tabuchi
authors.affiliation: Research Center for Advanced Science and Technology (RCAST), The University of Tokyo, Meguro-ku, Tokyo 153-8904, Japan.
authors.forename: Seiichiro
authors.initials: S
authors.lastname: Ishino
authors.affiliation: Research Center for Advanced Science and Technology (RCAST), The University of Tokyo, Meguro-ku, Tokyo 153-8904, Japan.
authors.forename: Toyofumi
authors.initials: T
authors.lastname: Ishikawa
authors.affiliation: Research Center for Advanced Science and Technology (RCAST), The University of Tokyo, Meguro-ku, Tokyo 153-8904, Japan.
authors.forename: Rekishu
authors.initials: R
authors.lastname: Yamazaki
authors.affiliation: Research Center for Advanced Science and Technology (RCAST), The University of Tokyo, Meguro-ku, Tokyo 153-8904, Japan.
authors.forename: Koji
authors.initials: K
authors.lastname: Usami
authors.affiliation: Research Center for Advanced Science and Technology (RCAST), The University of Tokyo, Meguro-ku, Tokyo 153-8904, Japan and Center for Emergent Matter Science (CEMS), RIKEN, Wako, Saitama 351-0198, Japan.
authors.forename: Yasunobu
authors.initials: Y
authors.lastname: Nakamura