Impact of Surface Treatment on Dielectric and Magnetic Defects in Superconducting Qubits

Strongly-coupled two-level state (TLS) defects lead to enhanced qubit energy relaxation rate at discrete operating frequencies. At the same time, magnetic interface defects give rise to low-frequency magnetic flux noise with a 1/f power spectrum, a dominant source of qubit dephasing. While some of these defects reside at the metal-substrate interface, others reside at the substrate-air or metal-air interfaces, and are thus amenable to modification by various surface treatments following device fabrication. Here, we use the technique of SWAP

spectroscopy to map out the spectrum of TLS that are strongly coupled to transmon qubits, and we use single-shot Ramsey interferometry to extract the power spectral density of 1/f flux noise in the qubit loop. We investigate various in situ surface treatments as a means to suppress both TLS and magnetic surface defects, and we explore the dependence of TLS density of states and flux noise power on qubit geometry.