Potential Nanoscale Sources of Decoherence in Niobium based Transmon Qubit Architectures

  1. Akshay A. Murthy,
  2. Paul Masih Das,
  3. Stephanie M. Ribet,
  4. Cameron Kopas,
  5. Jaeyel Lee,
  6. Matthew J. Reagor,
  7. Lin Zhou,
  8. Matthew J. Kramer,
  9. Mark C. Hersam,
  10. Mattia Checchin,
  11. Anna Grassellino,
  12. Roberto dos Reis,
  13. Vinayak P. Dravid,
  14. and Alexander Romanenko
Superconducting thin films of niobium have been extensively employed in transmon qubit architectures. Although these architectures have demonstrated remarkable improvements in recent years, further improvements in performance through materials engineering will aid in large-scale deployment. Here, we use information retrieved from electron microscopy and analysis to conduct a detailed assessment of potential decoherence sources in transmon qubit test devices. In the niobium thin film, we observe the presence of localized strain at interfaces, which may amplify interactions between two-level systems and impose limits on T1 and T2 relaxation times. Additionally, we observe the presence of a surface oxide with varying stoichiometry and bond distances, which can generate a broad two-level system noise spectrum. Finally, a similarly disordered and rough interface is observed between Nb and the Si substrate. We propose that this interface can also degrade the overall superconducting properties.
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