Resolving and Engineering the Surface Chemistry of Superconducting Quantum Circuits

Superconducting qubits have emerged as one of the leading hardware platforms for realizing a quantum processor. Consequently, researchers have made significant effort to understand the loss channels that limit the coherence times of qubits made with superconducting materials. A major source of loss has been attributed to two level systems (TLS) that are present at the material interfaces in superconducting qubits. Recently, it has been shown that by substituting the superconductor niobium with tantalum, coherence times of superconducting qubits can be improved significantly. In this talk, we will discuss our studies on resolving the surface/interfacial chemical profiles at the metal-air and metal-substrate interfaces of superconducting tantalum films, to identify possible sources of TLS losses. The talk will cover the study of surface oxidation of tantalum using a combination of Hard X-ray Photoelectron Spectroscopy (HAXPES) and scanning transmission electron microscopy - electron energy loss spectroscopy (STEM-EELS), from which we can quantatively determine the oxidation profile and atomic motifs across the ~3 nm-thick native oxide layer. We will also discuss the study of interfacial diffusion at the metal/substrate interface by synchrotron X-ray reflectivity. In the end, we will discuss a few surface and bulk engineering techniques that are used to achieve more perfect interfaces for less contribution to TLS loss.