Nb encapsulation studies for improved superconducting qubit coherence

A thin naturally forming layer of surface oxides of Nb is believed to be a significant source of decoherence in Nb based superconducting qubits. Here, we coat the Nb surface with 5 to 10 nm of passivation layer to inhibit the formation of Nb oxides. A wide range of materials has been explored as the passivation layer to include Al, Ta, and TiN, to name a few. We report the results of microwave loss measurements of superconducting transmission line resonators and coherence measurements of transmon qubit devices. Preliminary results indicate a correlation between microwave loss and coherence measurements. Supported by materials characterization tools, a systematic investigation is launched to help identify causes of microwave loss and decoherence, and converge on materials/passivations to mitigate loss mechanisms. This material is based upon work supported by the U.S. Department of Energy, Office of Science, National Quantum Information Science Research Centers, Superconducting Quantum Materials and Systems Center (SQMS) under contract number DE-AC02-07CH11359.