Low-photon number studies of inductively-coupled superconducting resonators*

Quality factors near one million have been observed in on-chip superconducting resonators for many years, but new studies on resonators reveal much lower quality factors at low-photon numbers, perhaps due to the presence of anomalous two-level system defects. We have designed and fabricated four new aluminum thin-film resonator types near 6 GHz. They include a lumped-element resonator, a slot-line resonator, and two hybrids, both of which contain a slot line and either an inductor or a capacitor. The resonator types have a consistent line width and are fabricated on a sapphire substrate to facilitate the study of surface defects, such as two-level systems. We plan to compare their quality factors in an effort to better understand the loss mechanism associated with the surface. All the resonator types have inductive coupling to a coplanar waveguide with geometrical symmetry that can be used to construct useful Josephson junction resonators.