Extracting Contributions to Qubit Loss from Superconducting Microwave Resonators. Part 1

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 Qi 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, Qi 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 Qi as a function of photon occupation in Al and Nb thin-film microwave resonators. We present data from devices that involve systematic variation of geometry in order to modify the participation ratios of lossy interfaces and determine the dominant contributors to energy relaxation.