Yulong Li

High-Precision Calibration Workflow Achieves Above 99.9% CZ Gate Fidelity on a Scalable Superconducting Processor

  1. Huili Zhang,
  2. Meiling Li,
  3. Shuang Yang,
  4. Yaqing Feng,
  5. Yulong Li,
  6. Cheng Chen,
  7. Pei Liu,
  8. Guangming Xue,
  9. and Haifeng Yu
High-fidelity universal two-qubit gates are critical for building fault-tolerant quantum computers. In scalable superconducting processors, shortened coherence times introduce more
incoherent errors in gate operations. With a constrained error budget, there is reduced tolerance for coherent errors stemming from parameter deviations. In this work, we develop a closed-loop workflow to enhance the CZ gate calibration precision. Utilizing the echoed leakage error amplification (ELEA) and the repurposed context-aware fidelity estimation (CAFE) circuits, we suppress the population leakage to non-computational states, and, for the first time, demonstrate a CZ gate fidelity exceeding 99.9% on an 84-qubit processor, with coherent error suppressed to 0.007%. Meanwhile, we obtain a median fidelity of 99.25% among 72 CZ gates, demonstrating that the workflow can be generalized to the calibration of parallel CZ gates. Finally, we realize automated calibration and observe enhanced stability of the CZ gate throughout 9-hour comparative monitoring experiments. Our results, realized on a completely domestic platform, establish an efficient and automated route to quantum computation with superconducting quantum systems.
arxiv pdf

Converting qubit relaxation into erasures with a single fluxonium

  1. Chenlu Liu,
  2. Yulong Li,
  3. Jiahui Wang,
  4. Quan Guan,
  5. Lijing Jin,
  6. Lu Ma,
  7. Ruizi Hu,
  8. Tenghui Wang,
  9. Xing Zhu,
  10. Hai-Feng Yu,
  11. Chunqing Deng,
  12. and Xizheng Ma
Qubits that experience predominantly erasure errors offer distinct advantages for fault-tolerant operation. Indeed, dual-rail encoded erasure qubits in superconducting cavities and
transmons have demonstrated high-fidelity operations by converting physical-qubit relaxation into logical-qubit erasures, but this comes at the cost of increased hardware overhead and circuit complexity. Here, we address these limitations by realizing erasure conversion in a single fluxonium operated at zero flux, where the logical state is encoded in its 0-2 subspace. A single, carefully engineered resonator provides both mid-circuit erasure detection and end-of-line (EOL) logical measurement. Post-selection on non-erasure outcomes results in more than four-fold increase of the logical lifetime, from 193 μs to 869 μs. Finally, we characterize measurement-induced logical dephasing as a function of measurement power and frequency, and infer that each erasure check contributes a negligible error of 7.2×10−5. These results establish integer-fluxonium as a promising, resource-efficient platform for erasure-based error mitigation, without requiring additional hardware.
arxiv pdf