Complex temperature dependence of coupling and dissipation of cavity-magnon polaritons from milliKelvin to room temperature

TL;DR

This study explores the temperature-dependent behavior of cavity-magnon polaritons in a YIG sphere from room temperature to millikelvin, revealing complex coupling and dissipation dynamics relevant for quantum information technologies.

Contribution

It provides comprehensive experimental data on the temperature dependence of coupling strength and dissipation in cavity-magnon systems from classical to quantum regimes.

Findings

Strong deviations in coupling strength at low temperatures.

Peak in magnonic dissipation at 40 K due to impurity scattering.

High cooperativity exceeding 100 at millikelvin and room temperature.

Abstract

Hybridized magnonic-photonic systems are key components for future information processing technologies such as storage, manipulation or conversion of data both in the classical (mostly at room temperature) and quantum (cryogenic) regime. In this work, we investigate a YIG sphere coupled strongly to a microwave cavity over the full temperature range from $290\,\mathrm{K}$ down to $30\,\mathrm{mK}$. The cavity-magnon polaritons are studied from the classical to the quantum regime where the thermal energy is less than one resonant microwave quanta, i.e. at temperatures below $1\,\mathrm{K}$. We compare the temperature dependence of the coupling strength $g_{\rm{eff}}(T)$, describing the strength of coherent energy exchange between spin ensemble and cavity photon, to the temperature behavior of the saturation magnetization evolution $M_{\rm{s}}(T)$ and find strong deviations at low temperatures. The temperature dependence of magnonic disspation is governed at intermediate temperatures by rare earth impurity scattering leading to a strong peak at $40\,$K. The linewidth $\kappa_{\rm{m}}$ decreases to $1.2\,$MHz at $30\,$mK, making this system suitable as a building block for quantum electrodynamics experiments. We achieve an electromagnonic cooperativity in excess of $20$ over the entire temperature range, with values beyond $100$ in the milliKelvin regime as well as at room temperature. With our measurements, spectroscopy on strongly coupled magnon-photon systems is demonstrated as versatile tool for spin material studies over large temperature ranges. Key parameters are provided in a single measurement, thus simplifying investigations significantly.