Probing Strong Coupling between a Microwave Cavity and a Spin Ensemble with Raman Heterodyne Spectroscopy

TL;DR

This paper demonstrates the use of Raman heterodyne spectroscopy to probe strong coupling between erbium spins and a microwave cavity, revealing hybrid modes and measuring long spin relaxation times relevant for quantum state conversion.

Contribution

It introduces a novel application of Raman heterodyne spectroscopy to strongly coupled spin-cavity systems and reports the first observation of hybrid modes and long spin relaxation times in this context.

Findings

Observation of Raman heterodyne signals at hybrid mode frequencies

Measurement of spin relaxation time T₁ = 10±3 seconds

Detection of signals from excited state spin transitions

Abstract

Raman heterodyne spectroscopy is a powerful tool for characterizing the energy and dynamics of spins. The technique uses an optical pump to transfer coherence from a spin transition to an optical transition where the coherent emission is more easily detected. Here Raman heterodyne spectroscopy is used to probe an isotopically purified ensemble of erbium dopants, in a yttrium orthosilicate (Y$_2$SiO$_5$) crystal coupled to a microwave cavity. Because the erbium electron spin transition is strongly coupled to the microwave cavity, we observed Raman heterodyne signals at the resonant frequencies of the hybrid spin-cavity modes (polaritons) rather than the bare erbium spin transition frequency. Using the coupled system, we made saturation recovery measurements of the ground state spin relaxation time T$_1$ = 10$\pm$3 seconds, and also observed Raman heterodyne signals using an excited state spin transition. We discuss the implications of these results for efforts towards converting microwave quantum states to optical quantum states.