Systematic improvements in transmon qubit coherence enabled by niobium surface encapsulation

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…

• NS
  National Science Foundation

  Awards: 2025633, DE-AC02-07CH11358, ECCS-2025633, DE-AC02-07CH11359
• UD
  U.S. Department of Energy

  Awards: AC02-07CH11358, DE-AC02-07CH11359, AC02-07CH11359, DE-AC02, DE-AC02-07CH11358, DE-AC02-
• UO
  University of Chicago
• IS
  Iowa State University

  Awards: 07CH11358, DE-AC02-07CH11358
• OO
  Office of Science

  Awards: AC02-07CH11358, DE-AC02, DE-AC02-07CH11359, DE-AC02-07CH11358
• DO
  Division of Electrical, Communications and Cyber Systems

  Awards: ECCS-2025633, 2025633