# Experimental Implementations of Cavity-Magnon Systems: from Ultra Strong Coupling to Applications in Precision Measurement

## TL;DR

This paper presents experimental implementations of cavity-magnon systems demonstrating ultra strong coupling regimes, exploring their potential for quantum technologies and dark matter detection, and engineering systems for enhanced stability against magnetic fluctuations.

## Contribution

It introduces novel cavity-magnon setups achieving ultra strong coupling and demonstrates their applications in precision measurement and dark matter detection.

## Key findings

- Achieved ultra strong coupling with g/ω=0.46 in YIG re-entrant cavity.
- Achieved ultra strong coupling with g/ω=0.34 in YIG loop gap cavity.
- Engineered a system with reduced sensitivity to magnetic field fluctuations.

## Abstract

Several experimental implementations of cavity-magnon systems are presented. First an Yttrium Iron Garnet (YIG) block is placed inside a re-entrant cavity where the resulting hybrid mode is measured to be in the ultra strong coupling regime. When fully hybridised the ratio between the coupling rate and uncoupled mode frequencies is determined to be $g/\omega=0.46$. Next a thin YIG cylinder is placed inside a loop gap cavity. The bright mode of this cavity couples to the YIG sample and is similarly measured to be in the ultra strong coupling regime with ratio of coupling rate to uncoupled mode frequencies as $g/\omega=0.34$. A larger spin density medium such as lithium ferrite (LiFe) is expected to improve couplings by a factor of 1.46 in both systems as coupling strength is shown to be proportional to the square root of spin density and magnetic moment. Such strongly coupled systems are potentially useful for cavity QED, hybrid quantum systems and precision dark matter detection experiments. The YIG disc in the loop gap cavity, is, in particular, shown to be a strong candidate for dark matter detection. Finally, a LiFe sphere inside a two post re-entrant cavity is considered. In past work it was shown that the magnon mode in the sample has a turnover point in frequency. Additionally, it was predicted that if the system was engineered such that it fully hybridised at this turnover point the cavity-magnon polariton (CMP) transition frequency would become insensitive to both first and second order magnetic bias field fluctuations, a result useful for precision frequency applications. This work implements such a system by engineering the cavity mode frequency to near this turnover point, with suppression in sensitivity to second order bias magnetic field fluctuations shown.