Improving qubit performance through engineering of the substrate-Josephson junction interface.*

The performance of a superconducting qubit is often limited by dissipation and two-level systems (TLS) losses. The dominant sources of these losses are believed to originate from interfaces and surfaces, likely as a result of fabrication processes, materials, or atmospheric exposure. We show an example of a chemical surface treatment that modifies the Josephson Junction-substrate interface, resulting in statistically significant improvement in T1 (>25% improvement) and a reduction in the density of strongly-coupled TLS. As this treatment primarily modifies the Josephson junction-substrate interface, we perform targeted chemical, and structural characterizations of that interface and correlate those with qubit measurements.