Superstrong Coupling of a Microwave Cavity to YIG Magnons

Nikita Kostylev, Maxim Goryachev, Michael E. Tobar

Published 2015-08-20Version 1

Multiple-post reentrant 3D lumped cavity modes have been realized to design the new concept of discrete Whispering Gallery and Fabry-Perot Modes for multimode microwave Quantum Electrodynamics experiments. Using a magnon spin-wave resonance of a submillimeter-sized Yttrium-Iron- Garnet sphere at millikelvin temperatures and a four-post cavity, we demonstrate the ultra-strong coupling regime between discrete Whispering Gallery Modes and a magnon resonance with strength of 1.84 GHz. By increasing the number of posts to eight and arranging them in a D4 symmetry pattern, we expand the mode structure to that of a discrete Fabry-Perot cavity and show control of the Free Spectral Range (FSR). We reach the superstrong coupling regime, where spin-photon coupling strength is larger than FSR, with coupling strength in the 1.1 to 1.5 GHz range.