Faranak Bahrami

2D transmons with lifetimes and coherence times exceeding 1 millisecond

  1. Matthew P. Bland,
  2. Faranak Bahrami,
  3. Jeronimo G.C. Martinez,
  4. Paal H. Prestegaard,
  5. Basil M. Smitham,
  6. Atharv Joshi,
  7. Elizabeth Hedrick,
  8. Alex Pakpour-Tabrizi,
  9. Shashwat Kumar,
  10. Apoorv Jindal,
  11. Ray D. Chang,
  12. Ambrose Yang,
  13. Guangming Cheng,
  14. Nan Yao,
  15. Robert J. Cava,
  16. Nathalie P. de Leon,
  17. and Andrew A. Houck
Materials improvements are a powerful approach to reducing loss and decoherence in superconducting qubits because such improvements can be readily translated to large scale processors.
Recent work improved transmon coherence by utilizing tantalum (Ta) as a base layer and sapphire as a substrate. The losses in these devices are dominated by two-level systems (TLSs) with comparable contributions from both the surface and bulk dielectrics, indicating that both must be tackled to achieve major improvements in the state of the art. Here we show that replacing the substrate with high-resistivity silicon (Si) dramatically decreases the bulk substrate loss, enabling 2D transmons with time-averaged quality factors (Q) exceeding 1.5 x 10^7, reaching a maximum Q of 2.5 x 10^7, corresponding to a lifetime (T_1) of up to 1.68 ms. This low loss allows us to observe decoherence effects related to the Josephson junction, and we use improved, low-contamination junction deposition to achieve Hahn echo coherence times (T_2E) exceeding T_1. We achieve these material improvements without any modifications to the qubit architecture, allowing us to readily incorporate standard quantum control gates. We demonstrate single qubit gates with 99.994% fidelity. The Ta-on-Si platform comprises a simple material stack that can potentially be fabricated at wafer scale, and therefore can be readily translated to large-scale quantum processors.
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