Xiaohang Zhang

Titanium Nitride Film on Sapphire Substrate with Low Dielectric Loss for Superconducting Qubits

  1. Hao Deng,
  2. Zhijun Song,
  3. Ran Gao,
  4. Tian Xia,
  5. Feng Bao,
  6. Xun Jiang,
  7. Hsiang-Sheng Ku,
  8. Zhisheng Li,
  9. Xizheng Ma,
  10. Jin Qin,
  11. Hantao Sun,
  12. Chengchun Tang,
  13. Tenghui Wang,
  14. Feng Wu,
  15. Wenlong Yu,
  16. Gengyan Zhang,
  17. Xiaohang Zhang,
  18. Jingwei Zhou,
  19. Xing Zhu,
  20. Yaoyun Shi,
  21. Hui-Hai Zhao,
  22. and Chunqing Deng
Dielectric loss is one of the major decoherence sources of superconducting qubits. Contemporary high-coherence superconducting qubits are formed by material systems mostly consisting
of superconducting films on substrate with low dielectric loss, where the loss mainly originates from the surfaces and interfaces. Among the multiple candidates for material systems, a combination of titanium nitride (TiN) film and sapphire substrate has good potential because of its chemical stability against oxidization, and high quality at interfaces. In this work, we report a TiN film deposited onto sapphire substrate achieving low dielectric loss at the material interface. Through the systematic characterizations of a series of transmon qubits fabricated with identical batches of TiN base layers, but different geometries of qubit shunting capacitors with various participation ratios of the material interface, we quantitatively extract the loss tangent value at the substrate-metal interface smaller than 8.9×10−4 in 1-nm disordered layer. By optimizing the interface participation ratio of the transmon qubit, we reproducibly achieve qubit lifetimes of up to 300 μs and quality factors approaching 8 million. We demonstrate that TiN film on sapphire substrate is an ideal material system for high-coherence superconducting qubits. Our analyses further suggest that the interface dielectric loss around the Josephson junction part of the circuit could be the dominant limitation of lifetimes for state-of-the-art transmon qubits.
arxiv pdf

Fluxonium: an alternative qubit platform for high-fidelity operations

  1. Feng Bao,
  2. Hao Deng,
  3. Dawei Ding,
  4. Ran Gao,
  5. Xun Gao,
  6. Cupjin Huang,
  7. Xun Jiang,
  8. Hsiang-Sheng Ku,
  9. Zhisheng Li,
  10. Xizheng Ma,
  11. Xiaotong Ni,
  12. Jin Qin,
  13. Zhijun Song,
  14. Hantao Sun,
  15. Chengchun Tang,
  16. Tenghui Wang,
  17. Feng Wu,
  18. Tian Xia,
  19. Wenlong Yu,
  20. Fang Zhang,
  21. Gengyan Zhang,
  22. Xiaohang Zhang,
  23. Jingwei Zhou,
  24. Xing Zhu,
  25. Yaoyun Shi,
  26. Jianxin Chen,
  27. Hui-Hai Zhao,
  28. and Chunqing Deng
Superconducting qubits provide a promising path toward building large-scale quantum computers. The simple and robust transmon qubit has been the leading platform, achieving multiple
milestones. However, fault-tolerant quantum computing calls for qubit operations at error rates significantly lower than those exhibited in the state of the art. Consequently, alternative superconducting qubits with better error protection have attracted increasing interest. Among them, fluxonium is a particularly promising candidate, featuring large anharmonicity and long coherence times. Here, we engineer a fluxonium-based quantum processor that integrates high qubit-coherence, fast frequency-tunability, and individual-qubit addressability for reset, readout, and gates. With simple and fast gate schemes, we achieve an average single-qubit gate fidelity of 99.97% and a two-qubit gate fidelity of up to 99.72%. This performance is comparable to the highest values reported in the literature of superconducting circuits. Thus our work, for the first time within the realm of superconducting qubits, reveals an approach toward fault-tolerant quantum computing that is alternative and competitive to the transmon system.
arxiv pdf