Reduction of microwave losses in superconducting circuits through ruthenium capping

Single qubit coherence times remain a bottleneck for scaling up superconducting qubit-based quantum processors. Niobium superconducting circuits offer high superconducting gaps, minimizing losses due to thermal quasiparticle at typical operating temperatures. However, losses due to two-level systems (TLSs) resided in the amorphous native oxide layer continue to limit the performance. To address this issue, we introduce an approach using an in-situ ruthenium capping layer to mitigate TLS-induced losses. We are able to deposit thin continuous films of ruthenium, preventing the formation of lossy Nb surface oxides when exposed to air. By maintaining the same superconducting resonator geometries and only varying the surface, we systematically investigate the impact of the ruthenium capping layer on TLS loss of Nb resonators.