High Q-factor Sapphire Whispering Gallery Mode Microwave Resonator at   Single Photon Energies and milli-Kelvin Temperatures
  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. 
Daniel L. Creedon, Yarema Reshitnyk, Warrick Farr, John M. Martinis,   Timothy L. Duty, Michael E. Tobar
quant-ph cond-mat.other