Hybridizing ferromagnetic magnons and microwave photons in the quantum limit

  1. Yutaka Tabuchi,
  2. Seiichiro Ishino,
  3. Toyofumi Ishikawa,
  4. Rekishu Yamazaki,
  5. Koji Usami,
  6. and Yasunobu Nakamura
We demonstrate large normal 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 numbers of thermally and externally excited magnons and photons are less than one. We also confirm that the coupling strength is proportional to the square root of the number of spins. Non-monotonous 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.
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