Nana Shumiya

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

Disentangling Losses in Tantalum Superconducting Circuits

  1. Kevin D. Crowley,
  2. Russell A. McLellan,
  3. Aveek Dutta,
  4. Nana Shumiya,
  5. Alexander P.M. Place,
  6. Xuan Hoang Le,
  7. Youqi Gang,
  8. Trisha Madhavan,
  9. Nishaad Khedkar,
  10. Yiming Cady Feng,
  11. Esha A. Umbarkar,
  12. Xin Gui,
  13. Lila V. H. Rodgers,
  14. Yichen Jia,
  15. Mayer M. Feldman,
  16. Stephen A. Lyon,
  17. Mingzhao Liu,
  18. Robert J. Cava,
  19. Andrew A. Houck,
  20. and Nathalie P. de Leon
Superconducting qubits are a leading system for realizing large scale quantum processors, but overall gate fidelities suffer from coherence times limited by microwave dielectric loss.
Recently discovered tantalum-based qubits exhibit record lifetimes exceeding 0.3 ms. Here we perform systematic, detailed measurements of superconducting tantalum resonators in order to disentangle sources of loss that limit state-of-the-art tantalum devices. By studying the dependence of loss on temperature, microwave photon number, and device geometry, we quantify materials-related losses and observe that the losses are dominated by several types of saturable two level systems (TLSs), with evidence that both surface and bulk related TLSs contribute to loss. Moreover, we show that surface TLSs can be altered with chemical processing. With four different surface conditions, we quantitatively extract the linear absorption associated with different surface TLS sources. Finally, we quantify the impact of the chemical processing at single photon powers, the relevant conditions for qubit device performance. In this regime we measure resonators with internal quality factors ranging from 5 to 15 x 10^6, comparable to the best qubits reported. In these devices the surface and bulk TLS contributions to loss are comparable, showing that systematic improvements in materials on both fronts will be necessary to improve qubit coherence further.
arxiv pdf