Systematic improvements in transmon qubit coherence enabled by niobium surface encapsulation
Mustafa Bal, Akshay A. Murthy, Shaojiang Zhu, Francesco Crisa, Xinyuan You, Ziwen Huang, Tanay Roy, Jaeyel Lee, David Van Zanten, Roman Pilipenko, Ivan Nekrashevich, Andrei Lunin, Daniel Bafia, Yulia Krasnikova, Cameron Kopas, Ella Lachman, Duncan Miller, Josh Mutus, Matthew J. Reagor, Hilal Cansizoglu, Jayss Marshall, David P. Pappas, Kim Vu, Kameshwar Yadavalli, Jin‐Su Oh, Lin Zhou, M. J. Kramer, Florent Lecocq, Dominic P. Goronzy, Carlos G. Torres‐Castanedo, P. Graham Pritchard, Vinayak P. Dravid, James M. Rondinelli, Michael J. Bedzyk, Mark C. Hersam, J. F. Zasadzinski, Jens Koch, J. A. Sauls, Alexander Romanenko, Anna Grassellino
Abstract
Abstract We present a transmon qubit fabrication technique that yields systematic improvements in T 1 relaxation times. We encapsulate the surface of niobium and prevent the formation of its lossy surface oxide. By maintaining the same superconducting metal and only varying the surface, this comparative investigation examining different capping materials, such as tantalum, aluminum, titanium nitride, and gold, as well as substrates across different qubit foundries demonstrates the detrimental impact that niobium oxides have on coherence times of superconducting qubits, compared to native oxides of tantalum, aluminum or titanium nitride. Our surface-encapsulated niobium qubit devices exhibit T 1 relaxation times 2–5 times longer than baseline qubit devices with native niobium oxides. When capping niobium with tantalum, we obtain median qubit lifetimes above 300 μs, with maximum values up to 600 μs. Our comparative structural and chemical analysis provides insight into why amorphous niobium oxides may induce higher losses compared to other amorphous oxides.