Yen-Chun Chen

Characterizing and Mitigating Flux Crosstalk in Superconducting Qubits-Couplers System

  1. Chen-Hsun Ma,
  2. Myrron Albert Callera Aguila,
  3. Nien-Yu Li,
  4. Li-Chieh Hsiao,
  5. Yi-Shiang Huang,
  6. Yen-Chun Chen,
  7. Teik-Hui Lee,
  8. Chin-Chia Chang,
  9. Jyh-Yang Wang,
  10. Ssu-Yen Huang,
  11. Hsi-Sheng Goan,
  12. Chiao-Hsuan Wang,
  13. Cen-Shawn Wu,
  14. Chii-Dong Chen,
  15. and Chung-Ting Ke
Superconducting qubits have achieved exceptional gate fidelities, exceeding the error-correction threshold in recent years. One key ingredient of such improvement is the introduction
of tunable couplers to control the qubit-to-qubit coupling through frequency tuning. Moving toward fault-tolerant quantum computation, increasing the number of physical qubits is another step toward effective error correction codes. Under a multiqubit architecture, flux control (Z) lines are crucial in tuning the frequency of the qubits and couplers. However, dense flux lines result in magnetic flux crosstalk, wherein magnetic flux applied to one element inadvertently affects neighboring qubits or couplers. This crosstalk obscures the idle frequency of the qubit when flux bias is applied, which degrades gate performance and calibration accuracy. In this study, we characterize flux crosstalk and suppress it in a multiqubit-coupler chip with multi-Z lines without adding additional readout for couplers. By quantifying the mutual flux-induced frequency shifts of qubits and couplers, we construct a cancellation matrix that enables precise compensation of non-local flux, demonstrating a substantial reduction in Z-line crosstalk from 56.5permilleto 0.13permille which is close to statistical error. Flux compensation corrects the CZ SWAP measurement, leading to a symmetric map with respect to flux bias. Compared with a crosstalk-free calculated CZ SWAP map, the measured map indicates that our approach provides a near-zero crosstalk for the coupler-transmon system. These results highlight the effectiveness of our approach in enhancing flux crosstalk-free control and supporting its potential for scaling superconducting quantum processors.
arxiv pdf

Scaffold-Assisted Window Junctions for Superconducting Qubit Fabrication

  1. Chung-Ting Ke,
  2. Jun-Yi Tsai,
  3. Yen-Chun Chen,
  4. Zhen-Wei Xu,
  5. Elam Blackwell,
  6. Matthew A. Snyder,
  7. Spencer Weeden,
  8. Peng-Sheng Chen,
  9. Chih-Ming Lai,
  10. Shyh-Shyuan Sheu,
  11. Zihao Yang,
  12. Cen-Shawn Wu,
  13. Alan Ho,
  14. R. McDermott,
  15. John Martinis,
  16. and Chii-Dong Chen
The superconducting qubit is one of the promising directions in realizing fault-tolerant quantum computing (FTQC), which requires many high-quality qubits. To achieve this, it is desirable
to leverage modern semiconductor industry technology to ensure quality, uniformity, and reproducibility. However, conventional Josephson junction fabrication relies mainly on resist-assistant double-angle evaporation, posing integration challenges. Here, we demonstrate a lift-off-free qubit fabrication that integrates seamlessly with existing industrial technologies. This method employs a silicon oxide (SiO2) scaffold to define an etched window with a well-controlled size to form a Josephson junction. The SiO2, which has a large dielectric loss, is etched away in the final step using vapor HF leaving little residue. This Window junction (WJ) process mitigates the degradation of qubit quality during fabrication and allows clean removal of the scaffold. The WJ process is validated by inspection and Josephson junction measurement. The scaffold removal process is verified by measuring the quality factor of the resonators. Furthermore, compared to scaffolds fabricated by plasma-enhanced chemical vapor deposition (PECVD), qubits made by WJ through physical vapor deposition (PVD) achieve relaxation time up to 57μs. Our results pave the way for a lift-off-free qubit fabrication process, designed to be compatible with modern foundry tools and capable of minimizing damage to the substrate and material surfaces.
arxiv pdf