Strong Coupling between Whispering Gallery Modes and Chromium Ions in Ruby

TL;DR

This paper demonstrates strong coupling between whispering gallery modes and chromium spins in ruby at microwave frequencies, revealing complex interactions beyond traditional models and enabling potential quantum information applications.

Contribution

It reports the first observation of strong spin-photon coupling in a ruby WGM resonator with large tunability and complex interactions not captured by standard models.

Findings

Spin-photon coupling up to 610 MHz observed

System operates in the strong coupling regime

Interactions within the spin bath affect system behaviour

Abstract

We report the study of interactions between cavity photons and paramagnetic Cr$^{3+}$ spins in a ruby (Cr$^{3+}$:Al$_2$O$_3$) Whispering Gallery mode (WGM) resonator. Examining the system at microwave frequencies and millikelvin temperatures, spin-photon couplings up to 610 MHz or about 5% of photon energy are observed between the impurity spins and high quality factor ($Q > 10^5$) WGM. Large tunability and spin-spin interaction allows operation in the strong coupling regime. The system exhibits behaviour not predicted by the usual Tavis-Cummings model because of interactions within the two-level spin bath, and the existence of numerous photonic modes.