Strong coupling of a Gd$^{3+}$ multilevel spin system to an on-chip superconducting resonator

TL;DR

This paper demonstrates strong coupling between a Gd$^{3+}$ spin ensemble in a crystal and a superconducting resonator, revealing significant spin-photon interactions and potential for quantum state control.

Contribution

It reports the first realization of strong coupling with a multilevel Gd$^{3+}$ spin system on-chip, including detailed modeling and cavity sensing analysis.

Findings

Achieved a spin-photon state separation of 146 MHz.

Estimated an average spin-photon coupling strength of approximately 620 Hz.

Observed perturbations in the crystal ground state due to cavity photons.

Abstract

We report the realization of a strong coupling between a Gd$^{3+}$ spin ensemble hosted in a scheelite (CaWO$_4$) single crystal and the resonant mode of a coplanar stripline superconducting cavity leading to a large separation of spin-photon states of 146 MHz. The interaction is well described by the Dicke model and the crystal-field Hamiltonian of the multilevel spin system. We observe a change of the crystal-field parameters due to the presence of photons in the cavity that generates a significant perturbation of the crystal ground state. Using finite-element calculations, we numerically estimate the cavity sensing volume as well as the average spin-photon coupling strength of $g_0\approx$ 620 Hz. Lastly, the dynamics of the spin-cavity states are explored via pulsed measurements by recording the cavity ring-down signal as a function of pulse length and amplitude. The results indicate a potential method to initialize this multilevel system in its ground state via an active cooling process.