Hybridizing ferromagnetic magnons and microwave photons in the quantum limit

TL;DR

This paper demonstrates strong quantum coupling between ferromagnetic magnons and microwave photons in yttrium iron garnet, revealing temperature-dependent dissipation effects and confirming the proportionality of coupling strength to the square root of spin number.

Contribution

It reports the first observation of large normal-mode splitting in the quantum regime between magnons and photons in a ferromagnetic system.

Findings

Achieved strong coupling in the quantum regime with less than one excitation on average.

Confirmed the coupling strength scales with the square root of the number of spins.

Observed nonmonotonic temperature dependence of the Kittel-mode linewidth below 1 K.

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.