Encapsulating superconducting circuits to avoid oxide-related dielectric loss

Single qubit coherence remains a major limiting factor in building scalable processors based on superconducting qubits. Tantalum-based superconducting qubits have been recently discovered to enable long lifetimes and coherence times because of their chemical robustness and well-behaved surface oxide [1,2,3,4]. One of the remaining major sources of dielectric loss has been measured to be two-level systems (TLSs) residing in the amorphous native oxide layer [5,6]. One strategy for avoiding such loss is to eliminate the oxide layer entirely. In this presentation I will describe our approach to eliminate the oxide layer by encapsulating tantalum with a noble metal thin film. We are able to deposit thin continuous films that prevent oxide formation while being thin enough to be fully proximitized, and we see no additional quasiparticle poisoning. We systematically study the impact of the encapsulation layer on TLS loss by correlating resonator loss measurements with direct materials characterization.