Correlating Superconducting Qubit Performance Losses to Sidewall Near-Field Scattering via Terahertz Nanophotonics

  1. Richard H. J. Kim,
  2. Samuel J. Haeuser,
  3. Joong-Mok Park,
  4. Randall K. Chan,
  5. Jin-Su Oh,
  6. Thomas Koschny,
  7. Lin Zhou,
  8. Matthew J. Kramer,
  9. Akshay A. Murthy,
  10. Mustafa Bal,
  11. Francesco Crisa,
  12. Sabrina Garattoni,
  13. Shaojiang Zhu,
  14. Andrei Lunin,
  15. David Olaya,
  16. Peter Hopkins,
  17. Alex Romanenko,
  18. Anna Grassellino,
  19. and Jigang Wang
Elucidating dielectric losses, structural heterogeneity, and interface imperfections is critical for improving coherence in superconducting qubits. However, most diagnostics rely on
destructive electron microscopy or low-throughput millikelvin quantum measurements. Here, we demonstrate noninvasive terahertz (THz) nano-imaging/-spectroscopy of encapsulated niobium transmon qubits, revealing sidewall near-field scattering that correlates with qubit coherence. We further employ a THz hyperspectral line scan to probe dielectric responses and field participation at Al junction interfaces. These findings highlight the promise of THz near-field methods as a high-throughput proxy characterization tool for guiding material selection and optimizing processing protocols to improve qubit and quantum circuit performance.

Stress-induced omega phase transition in Nb thin films for superconducting qubits

  1. Jaeyel Lee,
  2. Zuhawn Sung,
  3. Akshay A. Murthy,
  4. Anna Grassellino,
  5. and Alex Romanenko
We report the observation of omega phase formation in Nb thin films deposited by high-power impulse magnetron sputtering (HiPIMS) for superconducting qubits using transmission electron
microscopy (TEM). We hypothesize that this phase transformation to the omega phase with hexagonal structure from bcc phase as well as the formation of {111}<112> mechanical twins is induced by internal stress in the Nb thin films. In terms of lateral dimensions, the size of the omega phase of Nb range from 10 to 100 nm, which is comparable to the coherence length of Nb (~40 nm). In terms of overall volume fraction, ~1 vol.% of the Nb grains exhibit this omega phase. We also find that the omega phase in Nb is not observed in large grain Nb samples, suggesting that the phase transition can be suppressed through reducing the grain boundary density, which may serve as a source of strain and dislocations in this system. The current finding may indicate that the Nb thin film is prone to the omega phase transition due to the internal stress in the Nb thin film. We conclude by discussing effects of the omega phase on the superconducting properties of Nb thin films and discussing pathways to mitigate their formation.