Improving the Quality Factor of Microwave Compact Resonators by   Optimizing their Geometrical Parameters
  Applications in quantum information processing and photon detectors are stimulating a race to produce the highest possible quality factor on-chip superconducting microwave resonators. We have tested the surface-dominated loss hypothesis by systematically studying the role of geometrical parameters on the internal quality factors of compact resonators patterned in Nb on sapphire. Their single-photon internal quality factors were found to increase with the distance between capacitor fingers, the width of the capacitor fingers, and the impedance of the resonator. Quality factors were improved from 210,000 to 500,000 at T = 200 mK. All of these results are consistent with our starting hypothesis. 
K. Geerlings, S. Shankar, E. Edwards, L. Frunzio, R. J. Schoelkopf, M.   H. Devoret
cond-mat.supr-con physics.ins-det