Zhaohua Yang

Low-Loss, High-Coherence Airbridge Interconnects Fabricated by Single-Step Lithography

  1. Jibang Fu,
  2. Bo Ren,
  3. Jiandong Ouyang,
  4. Cong Li,
  5. Kechengqi Zhu,
  6. Yonggang Che,
  7. Xiang Fu,
  8. Shichuan Xue,
  9. Zhaohua Yang,
  10. Mingtang Deng,
  11. and Junjie Wu
Airbridges are essential for creating high-performance, low-parasitic interconnects in integrated circuits and quantum devices. Conventional multi-step fabrication methods hinder miniaturization
and introduce process-related defects. We report a simplified process for fabricating nanoscale airbridges using only a single electron-beam lithography step. By optimizing a multilayer resist stack with a triple-exposure-dose scheme and a thermal reflow step, we achieve smooth, suspended metallic bridges with sub-200-nm features that exhibit robust mechanical stability. Fabricated within a gradiometric SQUID design for superconducting transmon qubits, these airbridges introduce no measurable additional loss in the relaxation time T1, while enabling a 2.5-fold enhancement of the dephasing time T∗2. This efficient method offers a practical route toward integrating high-performance three-dimensional interconnects in advanced quantum and nano-electronic devices.
arxiv pdf

Vacuum-gap transmon qubits realized using flip-chip technology

  1. Xuegang Li,
  2. Yingshan Zhang,
  3. Chuhong Yang,
  4. Zhiyuan Li,
  5. Junhua Wang,
  6. Tang Su,
  7. Mo Chen,
  8. Yongchao Li,
  9. Chengyao Li,
  10. Zhenyu Mi,
  11. Xuehui Liang,
  12. Chenlu Wang,
  13. Zhen Yang,
  14. Yulong Feng,
  15. Kehuan Linghu,
  16. Huikai Xu,
  17. Jiaxiu Han,
  18. Weiyang Liu,
  19. Peng Zhao,
  20. Teng Ma,
  21. Ruixia Wang,
  22. Jingning Zhang,
  23. Yu Song,
  24. Pei Liu,
  25. Ziting Wang,
  26. Zhaohua Yang,
  27. Guangming Xue,
  28. Yirong Jin,
  29. and Haifeng Yu
Significant progress has been made in building large-scale superconducting quantum processors based on flip-chip technology. In this work, we use the flip-chip technology to realize
a modified transmon qubit, donated as the „flipmon“, whose large shunt capacitor is replaced by a vacuum-gap parallel plate capacitor. To further reduce the qubit footprint, we place one of the qubit pads and a single Josephson junction on the bottom chip and the other pad on the top chip which is galvanically connected with the single Josephson junction through an indium bump. The electric field participation ratio can arrive at nearly 53% in air when the vacuum-gap is about 5 microns, and thus potentially leading to a lower dielectric loss. The coherence times of the flipmons are measured in the range of 30-60 microseconds, which are comparable with that of traditional transmons with similar fabrication processes. The electric field simulation indicates that the metal-air interface’s participation ratio increases significantly and may dominate the qubit’s decoherence. This suggests that more careful surface treatment needs to be considered. No evidence shows that the indium bumps inside the flipmons cause significant decoherence. With well-designed geometry and good surface treatment, the coherence of the flipmons can be further improved.
arxiv pdf