Improving quality factors of superconducting coplanar wave guide resonators by surface passivation with self-assembled monolayer*

Superconducting coplanar waveguide (CPW) microwave resonators are among the best to read and change the state of artificial atoms because of their excellent coupling to quantum systems. This coupling is the base of forming circuit quantum electrodynamics (cQED) architecture. CPW resonators are very sensitive to deleterious thin film amorphous defects in their surfaces mainly due the presence of two-level system (TLS) oxides and non-TLS quasiparticles that significantly decrease their quality factors. In this work, we show the characterization and fabrication of niobium CPW resonators with more than 10⁶ internal quality factors at single-photon-excitation power, measured at 100 mK, for a particular surface treatment using self-assembled monolayers (SAM) molecules. We also compare our measurement results with finite element simulations (COMSOL and HFSS) and show analysis of resonator samples using structural characterization tools (XPS and TEM) to correlate the efficiency of surface treatment with resonators quality factors.