Modeling geometric dependence of dielectric losses in superconducting coplanar-waveguide resonators

Superconducting coplanar-waveguide (CPW) resonators are essential devices in circuit quantum electrodynamics (cQED). Their performance is limited by dielectric losses in the substrate and in the thin lossy oxide layers on the material interfaces. Reliable modeling is required to aid in the design of low-loss CPW structures for cQED. We analyze the geometric dependence of the dielectric losses in CPW structures using finite-element modeling of the participation ratios of the lossy regions. Material and device specific parameters of these regions are generally not known accurately enough, introducing uncertainty in the simulations. To this end, we carry out simulations on a range of CPW geometries and parameters. Combining the simulations with measured two-level-system-limited Q factors of CPW resonators, we solve an inverse problem to find optimal model parameters producing these values. Utilizing this model and our geometric-dependence analysis, we predict high Q factors obtainable by optimizing the cross-sectional geometries of the measured CPW structures. Our results guide the fabrication of low-loss CPW resonators for cQED.