High Q-factor Sapphire Whispering Gallery Mode Microwave Resonator at Single Photon Energies and milli-Kelvin Temperatures

TL;DR

This study demonstrates that sapphire whispering gallery mode microwave resonators can achieve ultra-high Q-factors exceeding 10^8 at millikelvin temperatures and single photon energies, highlighting their potential for quantum applications.

Contribution

The paper provides the first measurement of sapphire whispering gallery mode resonators at single photon energies and millikelvin temperatures, showing their potential for quantum-limited microwave resonators.

Findings

Q-factors > 1e8 achieved at 30 mK

Loss tangent limited to about 2e-8 due to electron spin resonance

Power-dependent frequency shifts indicate magnetic susceptibility effects

Abstract

The microwave properties of a crystalline sapphire dielectric whispering gallery mode resonator have been measured at very low excitation strength (E/hf=1) and low temperatures (T = 30 mK). The measurements were sensitive enough to observe saturation due to a highly detuned electron spin resonance, which limited the loss tangent of the material to about 2e-8 measured at 13.868 and 13.259 GHz. Small power dependent frequency shifts were also measured which correspond to an added magnetic susceptibility of order 1e-9. This work shows that quantum limited microwave resonators with Q-factors > 1e8 are possible with the implementation of a sapphire whispering gallery mode system.