Yongchao Li

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

Transmon qubit with relaxation time exceeding 0.5 milliseconds

  1. Chenlu Wang,
  2. Xuegang Li,
  3. Huikai Xu,
  4. Zhiyuan Li,
  5. Junhua Wang,
  6. Zhen Yang,
  7. Zhenyu Mi,
  8. Xuehui Liang,
  9. Tang Su,
  10. Chuhong Yang,
  11. Guangyue Wang,
  12. Wenyan Wang,
  13. Yongchao Li,
  14. Mo Chen,
  15. Chengyao Li,
  16. Kehuan Linghu,
  17. Jiaxiu Han,
  18. Yingshan Zhang,
  19. Yulong Feng,
  20. Yu Song,
  21. Teng Ma,
  22. Jingning Zhang,
  23. Ruixia Wang,
  24. Peng Zhao,
  25. Weiyang Liu,
  26. Guangming Xue,
  27. Yirong Jin,
  28. and Haifeng Yu
By using the dry etching process of tantalum (Ta) film, we had obtained transmon qubit with the best lifetime (T1) 503 us, suggesting that the dry etching process can be adopted in
the following multi-qubit fabrication with Ta film. We also compared the relaxation and coherence times of transmons made with different materials (Ta, Nb and Al) with the same design and fabrication processes of Josephson junction, we found that samples prepared with Ta film had the best performance, followed by those with Al film and Nb film. We inferred that the reason for this difference was due to the different loss of oxide materials located at the metal-air interface.
arxiv pdf

Realisation of adiabatic and di-adiabatic CZ gates in superconducting qubits coupled with a tunable coupler

  1. Huikai Xu,
  2. Weiyang Liu,
  3. Zhiyuan Li,
  4. Jiaxiu Han,
  5. Jingning Zhang,
  6. Kehuan Linghu,
  7. Yongchao Li,
  8. Mo Chen,
  9. Zhen Yang,
  10. Junhua Wang,
  11. Teng Ma,
  12. Guangming Xue,
  13. Yirong Jin,
  14. and Haifeng Yu
High fidelity two-qubit gates are fundamental for scaling up the superconducting number. We use two qubits coupled via a frequency-tunable coupler which can adjust the coupling strength,
and demonstrate the CZ gate using two different schemes, adiabatic and di-adiabatic methods. The Clifford based Randomized Benchmarking (RB) method is used to assess and optimize the CZ gate fidelity. The fidelity of adiabatic and di-adiabatic CZ gates are 99.53(8)% and 98.72(2)%, respectively. We also analyze the errors induced by the decoherence, which are 92% of total for adiabatic CZ gate and 46% of total for di-adiabatic CZ gates. The adiabatic scheme is robust against the operation error. But the di-adiabatic scheme is sensitive to the purity and operation errors. Comparing to 30 ns duration time of adiabatic CZ gate, the duration time of di-adiabatic CZ gate is 19 ns, revealing lower incoherence error rincoherent,Clfford = 0.0197(5) than r′incoherent,Clfford = 0.0223(3).
arxiv pdf