Mitigation of Superconducting Quantum Coherent Losses by Surface Passivation with Self-Assembled Monolayers

Quality factor of superconducting coplanar waveguide (CPW) microwave resonators is directly related to quantum coherence of superconducting circuits. Long coherence time is one of the key factors in realizing a commercial scale quantum computer and other related devices. The unique coupling of CPW resonators to other elements in quantum circuits is what forms the base of circuit quantum electrodynamics (cQED) architecture. While extensive research has explored techniques to reduce coherent losses of such devices, the precise structure of amorphous dielectric layers on surfaces and interfaces and their associated losses mechanism remain topics of active discussion. This is due to the presence of two-level system (TLS) oxides and non-TLS quasiparticles. In this work we present the design, fabrication and characterization of Niobium CPW resonators with a particular surface treatment using self-assembled monolayers (SAMs) that result in reducing superconducting losses. We show SAM-passivated resonators with more than 10⁶ internal quality factors at single-photon-excitation power, measured at 100 mK, that have been probed using a suite of structural characterization tools (SEM, XPS and TEM) to show the efficiency of our surface treatment. We finally compare the improvements in quality factors to our numerical simulations.