High Q-factor sapphire whispering gallery mode microwave resonator at single photon energies and millikelvin temperatures

The microwave properties of a crystalline sapphire dielectric whispering gallery mode resonator have been measured at very low excitation strength (E/n omega approximate to 1) and low temperatures (T approximate to 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 2 x 10(-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 10(-9). This work shows that quantum limited microwave resonators with Q-factors >10(8) are possible with the implementation of a sapphire whispering gallery mode system. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3595942]