Extracting Contributions to Qubit Loss from Superconducting Microwave Resonators

Superconducting coplanar waveguide resonators play a critical role in information storage and qubit state measurement in superconducting quantum information processing. At the same time, these resonators are a versatile testbed for characterizing the various contributions to qubit loss. Ideally, the internal quality factors Q_i of these resonators should reach ten million or higher, limited only by the loss tangent of the silicon or sapphire substrate. However, in real devices, Q_i is limited by the presence of various loss channels, including two-level state (TLS) defects at amorphous interfaces, trapped magnetic flux vortices, and nonequilibrium quasiparticles. In this work, we measure Q_i as a function of photon occupation in Al and Nb thin-film microwave resonators with different center conductor and gap widths. This allows us to extract the contribution to loss from interfacial TLS defects. We have also implemented novel resonator designs, including tapered resonators to study the relative contributions to loss from TLS and quasiparticles, and resonators with flux pinning structures to suppress the loss contribution from trapped flux. We have characterized devices fabricated on different substrates, used several methods to deposit the Al and Nb films, and explored various surface cleaning techniques to remove the native silicon oxide immediately prior to cooling the resonators.