Hybridizing ferromagnetic magnons and microwave photons in the quantum limit

Phys Rev Lett. 2014 Aug 22;113(8):083603. doi: 10.1103/PhysRevLett.113.083603. Epub 2014 Aug 22.


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.