Formation of niobium hydride precipitates in superconducting qubits

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Abstract
We report evidence for the formation of niobium hydride phase within niobium films on silicon substrates in superconducting qubits fabricated at Rigetti Computing. For this study, we combined complementary techniques—including room-temperature and cryogenic atomic force microscopy (AFM), synchrotron x-ray diffraction, and time-of-flight secondary ion mass spectroscopy (ToF-SIMS)—to directly reveal the existence of niobium hydride precipitates on the surface of superconducting qubits. Upon cryogenic cooling, we observed variation in the size and morphology of the hydrides, ranging from small (∼5 nm) irregular shapes to large (∼10–100 nm) domains within the Nb grains, which were fully converted to niobium hydrides. Since niobium hydrides are nonsuperconducting and can easily change in size and location upon different cooldowns to cryogenic temperature, our finding highlights a previously unknown source of decoherence in superconducting qubits. This contributes to quasiparticle losses, offering a potential explanation for changes in qubit performance upon cooldowns. Finally, by leveraging the RF performance of a 3D bulk Nb resonator, we quantify RF dissipation in a superconducting qubit caused by hydrogen concentration variation, and propose a practical engineering pathway to mitigate the formation of Nb hydrides for superconducting qubit applications.
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Funding Information
  • U.S. Department of Energy (89243024CSC000002)
  • Office of Science
  • Ministerio de Ciencia, Innovación y Universidades
  • Consejo Superior de Investigaciones Científicas (2010 6 0E 013)
  • Consejo Superior de Investigaciones Científicas (2021 60 E 030)
  • Environmental Studies Research Fund
  • Northwestern University
  • National Science Foundation (NSF ECCS-1542205)
  • National Science Foundation (NSF DMR-3541121262)
  • International Institute for Nanotechnology, Northwestern University
  • W. M. Keck Foundation

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