Approaching to the deep-strong photon-to-magnon coupling

TL;DR

This paper demonstrates ultra-strong photon-to-magnon coupling in on-chip multilayered structures, revealing new quantum phenomena and challenging existing models by evidencing the significance of the A^2 interaction term.

Contribution

It reports the first on-chip ultra-strong photon-to-magnon coupling exceeding 6 GHz and highlights the importance of the A^2 term in the system's Hamiltonian.

Findings

Coupling strength above 6 GHz achieved

Evidence of the A^2 interaction term's contribution

Observation of plasmon-magnon polariton

Abstract

In this work, the ultra-strong photon-to-magnon coupling is demonstrated for on-chip multilayered superconductor/ferromagnet/insulator hybrid thin film structures reaching the coupling strength above 6 GHz, the coupling ratio about 0.6, the single-spin coupling strength about 350 Hz, and cooperativity about 10^4. High characteristics of coupling are achieved owing to a radical suppression of the photon phase velocity in electromagnetic resonator. With achieved coupling the spectrum reveals inapplicability of the Dicke model, and evidences contribution of the diamagnetic A^2 interaction term in the Hamiltonian of the system, which satisfies the Thomas-Reiche-Kuhn sum rule. The contribution of the A^2 term denotes validity of the Hopfield quantum model and manifests observation of a different hybrid polariton quasi-particle, namely, the plasmon-magnon polariton.