Chen-Hsun Ma

Suppression of Quasiparticle Poisoning to 10−11 Levels in Superconducting Qubits via Infrared Shielding

  1. Wei-En Lin,
  2. Chen-Hsun Ma,
  3. Erh-Hsiang Yeh,
  4. Wei-Lun Peng,
  5. Yu-Sen Wei,
  6. Hsi-Sheng Goan,
  7. Cen-Shawn Wu,
  8. Chung-Ting Ke,
  9. Yung-Fu Chen,
  10. and Chii-Dong Chen
Quasiparticle poisoning bottlenecks superconducting qubits, limiting coherence and the scalability of quantum processors. In this work, we systematically investigate quasiparticle poisoning
in superconducting qubits under three infrared (IR) shielding configurations, ranging from a dedicated multi-layer design to a simplified implementation. By measuring quasiparticle-induced parity switching, we demonstrate a suppression of the switching rate by over four orders of magnitude via the implementation of improved shielding. In the best configuration, the rate decreases over time following cooldown and reaches 0.069Hz on day 34, corresponding to an anticipated quasiparticle density per Cooper pair of 1.88×10−11. To our knowledge, this represents the lowest quasiparticle density reported in the literature to date. The remaining quasiparticle population is likely dominated by sporadic phonon bursts stemming from mechanical stress release in the on-chip films, as well as from the surrounding environment. The effective qubit temperature follows the phonon bath down to 17mK, enabling initialization errors of ∼0.01% for 3GHz qubits. These results demonstrate that proper IR shielding and thermalization are essential for suppressing quasiparticle poisoning and enabling high-coherence, scalable superconducting qubit systems.
arxiv pdf

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