Alexander C. Pakpour-Tabrizi

Quantifying surface losses in superconducting aluminum microwave resonators

  1. Elizabeth Hedrick,
  2. Faranak Bahrami,
  3. Alexander C. Pakpour-Tabrizi,
  4. Atharv Joshi,
  5. Q. Rumman Rahman,
  6. Ambrose Yang,
  7. Ray D. Chang,
  8. Matthew P. Bland,
  9. Apoorv Jindal,
  10. Guangming Cheng,
  11. Nan Yao,
  12. Robert J. Cava,
  13. Andrew A. Houck,
  14. and Nathalie P. de Leon
The recent realization of millisecond-scale coherence with tantalum-on-silicon transmon qubits showed that depositing the Al/AlOx/Al Josephson junction in a high purity, ultrahigh vacuum
environment was critical for achieving lifetime-limited coherence, motivating careful examination of the aluminum surface two-level system (TLS) bath. Here, we measure the microwave absorption arising from surface TLSs in superconducting aluminum resonators, following methodology developed for tantalum resonators. We vary film and surface properties and correlate microwave measurements with materials characterization. We find that the lifetimes of superconducting aluminum resonators are primarily limited by surface losses associated with TLSs in the 2.7 nm-thick native AlOx. Treatment with 49% HF removes surface AlOx completely; however, rapid oxide regrowth limits improvements in surface loss and long term device stability. Using these measurements we estimate that TLSs in aluminum interfaces contribute around 27% of the relaxation rate of state-of-the-art tantalum-on-silicon qubits that incorporate aluminum-based Josephson junctions.
arxiv pdf

Eliminating Surface Oxides of Superconducting Circuits with Noble Metal Encapsulation

  1. Ray D. Chang,
  2. Nana Shumiya,
  3. Russell A. McLellan,
  4. Yifan Zhang,
  5. Matthew P. Bland,
  6. Faranak Bahrami,
  7. Junsik Mun,
  8. Chenyu Zhou,
  9. Kim Kisslinger,
  10. Guangming Cheng,
  11. Alexander C. Pakpour-Tabrizi,
  12. Nan Yao,
  13. Yimei Zhu,
  14. Mingzhao Liu,
  15. Robert J. Cava,
  16. Sarang Gopalakrishnan,
  17. Andrew A. Houck,
  18. and Nathalie P. de Leon
The lifetime of superconducting qubits is limited by dielectric loss, and a major source of dielectric loss is the native oxide present at the surface of the superconducting metal.
Specifically, tantalum-based superconducting qubits have been demonstrated with record lifetimes, but a major source of loss is the presence of two-level systems (TLSs) in the surface tantalum oxide. Here, we demonstrate a strategy for avoiding oxide formation by encapsulating the tantalum with noble metals that do not form native oxide. By depositing a few nanometers of Au or AuPd alloy before breaking vacuum, we completely suppress tantalum oxide formation. Microwave loss measurements of superconducting resonators reveal that the noble metal is proximitized, with a superconducting gap over 80% of the bare tantalum at thicknesses where the oxide is fully suppressed. We find that losses in resonators fabricated by subtractive etching are dominated by oxides on the sidewalls, suggesting total surface encapsulation by additive fabrication as a promising strategy for eliminating surface oxide TLS loss in superconducting qubits.
arxiv pdf