Xuandong Sun

Noise-induced quantum synchronization and maximally entangled mixed states in superconducting circuits

  1. Ziyu Tao,
  2. Finn Schmolke,
  3. Chang-Kang Hu,
  4. Wenhui Huang,
  5. Yuxuan Zhou,
  6. Jiawei Zhang,
  7. Ji Chu,
  8. Libo Zhang,
  9. Xuandong Sun,
  10. Zecheng Guo,
  11. Jingjing Niu,
  12. Wenle Weng,
  13. Song Liu,
  14. Youpeng Zhong,
  15. Dian Tan,
  16. Dapeng Yu,
  17. and Eric Lutz
Random fluctuations can lead to cooperative effects in complex systems. We here report the experimental observation of noise-induced quantum synchronization in a chain of superconducting
transmon qubits with nearest-neighbor interactions. The application of Gaussian white noise to a single site leads to synchronous oscillations in the entire chain. We show that the two synchronized end qubits are entangled, with nonzero concurrence, and that they belong to a class of generalized Bell states known as maximally entangled mixed states, whose entanglement cannot be increased by any global unitary. We further demonstrate the stability against frequency detuning of both synchronization and entanglement by determining the corresponding generalized Arnold tongue diagrams. Our results highlight the constructive influence of noise in a quantum many-body system and uncover the potential role of synchronization for mixed-state quantum information science.
arxiv pdf

Coupler-Assisted Leakage Reduction for Scalable Quantum Error Correction with Superconducting Qubits

  1. Xiaohan Yang,
  2. Ji Chu,
  3. Zechen Guo,
  4. Wenhui Huang,
  5. Yongqi Liang,
  6. Jiawei Liu,
  7. Jiawei Qiu,
  8. Xuandong Sun,
  9. Ziyu Tao,
  10. Jiawei Zhang,
  11. Jiajian Zhang,
  12. Libo Zhang,
  13. Yuxuan Zhou,
  14. Weijie Guo,
  15. Ling Hu,
  16. Ji Jiang,
  17. Yang Liu,
  18. Xiayu Linpeng,
  19. Tingyong Chen,
  20. Yuanzhen Chen,
  21. Jingjing Niu,
  22. Song Liu,
  23. Youpeng Zhong,
  24. and Dapeng Yu
Superconducting qubits are a promising platform for building fault-tolerant quantum computers, with recent achievement showing the suppression of logical error with increasing code
size. However, leakage into non-computational states, a common issue in practical quantum systems including superconducting circuits, introduces correlated errors that undermine QEC scalability. Here, we propose and demonstrate a leakage reduction scheme utilizing tunable couplers, a widely adopted ingredient in large-scale superconducting quantum processors. Leveraging the strong frequency tunability of the couplers and stray interaction between the couplers and readout resonators, we eliminate state leakage on the couplers, thus suppressing space-correlated errors caused by population propagation among the couplers. Assisted by the couplers, we further reduce leakage to higher qubit levels with high efficiency (98.1%) and low error rate on the computational subspace (0.58%), suppressing time-correlated errors during QEC cycles. The performance of our scheme demonstrates its potential as an indispensable building block for scalable QEC with superconducting qubits.
arxiv pdf