Understanding Material Systems for Superconducting Qubits*

Superconducting qubits have emerged as one of the leading hardware platforms for realizing large scale quantum processors. Our group has recently demonstrated lifetimes in superconducting qubits made from tantalum which exceed the highest of those of qubits made from niobium, aluminum, and other materials by a factor of more than three. In this talk, I will discuss our recent work to characterize the dominant sources of loss in state-of-the-art tantalum superconducting circuits. Using systematic measurements of tantalum resonators, we find the dominant source of loss at qubit operating conditions is from two-level systems present at material interfaces and surfaces. We compare the losses across samples prepared with different surface treatments, and we use surface spectroscopy to characterize the microscopic structure of these surfaces. Our work points the way to strategies for mitigating loss, as well as new opportunities in novel material systems.