Planar Superconducting Resonators with Internal Quality Factors above   One Million
  We describe the fabrication and measurement of microwave coplanar waveguide resonators with internal quality factors above 10 million at high microwave powers and over 1 million at low powers, with the best low power results approaching 2 million, corresponding to ~1 photon in the resonator. These quality factors are achieved by controllably producing very smooth and clean interfaces between the resonators' aluminum metallization and the underlying single crystal sapphire substrate. Additionally, we describe a method for analyzing the resonator microwave response, with which we can directly determine the internal quality factor and frequency of a resonator embedded in an imperfect measurement circuit. 
A. Megrant, C. Neill, R. Barends, B. Chiaro, Yu Chen, L. Feigl, J.   Kelly, Erik Lucero, Matteo Mariantoni, P. J. J. O'Malle...
cond-mat.mes-hall cond-mat.mtrl-sci cond-mat.supr-con quant-ph