Zhengqi Niu

A plug-and-play superconducting quantum controller at millikelvin temperatures enables exceeding 99.9% average gate fidelity

  1. Kuang Liu,
  2. Zhiyuan Wang,
  3. Xiaoliang He,
  4. Siqi Li,
  5. Hao Wu,
  6. Xiangyu Ren,
  7. Zhengqi Niu,
  8. Wangpeng Gao,
  9. Chenluo Zhang,
  10. Pei Huang,
  11. Yu Wu,
  12. Liliang Ying,
  13. Wei Peng,
  14. Jaw-Shen Tsai,
  15. and Zhirong Lin
The development of large-scale superconducting quantum computing requires efficient in-situ control methods that allow high-fidelity operations at millikelvin temperatures. Superconducting
circuits based on Josephson junctions offer a promising solution due to their high speed, low power dissipation, and cryogenic nature. Here, we report a superconducting quantum controller that enables direct chip-to-chip interconnection with qubits at 10 mK and high-fidelity, all-digital manipulation. Randomized benchmarking reveals a uniformly high average Clifford fidelity of 99.9% with leakage to high energy levels on the order of 10−4, and an estimated average gate operation energy of 0.121 fJ, demonstrating the potential to resolve the control bottleneck in superconducting quantum computing.
arxiv pdf

Tunable frequency conversion and comb generation with a superconducting artificial atom

  1. Fahad Aziz,
  2. Zhengqi Niu,
  3. Tzu-Yen Hsieh,
  4. Kuan-Ting Lin,
  5. Yu-Huan Huang,
  6. Yen-Hsiang Lin,
  7. Ching-Yeh Chen,
  8. Yu-Ting Cheng,
  9. Kai-Min Hsieh,
  10. Jeng-Chung Chen,
  11. Anton Frisk Kockum,
  12. Guin-Dar Lin,
  13. Zhi-Rong Lin,
  14. Ping-Yi Wen,
  15. and Io-Chun Hoi
We investigate the power spectral density emitted by a superconducting artificial atom coupled to the end of a semi-infinite transmission line and driven by two continuous radio-frequency
fields. In this setup, we observe the generation of multiple frequency peaks and the formation of frequency combs with equal detuning between those peaks. The frequency peaks originate from wave mixing of the drive fields, mediated by the artificial atom, highlighting the potential of this system as both a frequency converter and a frequency-comb generator. We demonstrate precise control and tunability in generating these frequency features, aligning well with theoretical predictions, across a relatively wide frequency range (tens of MHz, exceeding the linewidth of the artificial atom). The extensive and simple tunability of this frequency converter and comb generator, combined with its small physical footprint, makes it promising for quantum optics on chips and other applications in quantum technology.
arxiv pdf

Quasiparticle Dynamics in Superconducting Quantum-Classical Hybrid Circuits

  1. Kuang Liu,
  2. Xiaoliang He,
  3. Zhengqi Niu,
  4. Hang Xue,
  5. Wenbing Jiang,
  6. Liliang Ying,
  7. Wei Peng,
  8. Masaaki Maezawa,
  9. Zhirong Lin,
  10. Xiaoming Xie,
  11. and Zhen Wang
Single flux quantum (SFQ) circuitry is a promising candidate for a scalable and integratable cryogenic quantum control system. However, the operation of SFQ circuits introduces non-equilibrium
quasiparticles (QPs), which are a significant source of qubit decoherence. In this study, we investigate QP behavior in a superconducting quantum-classical hybrid chip that comprises an SFQ circuit and a qubit circuit. By monitoring qubit relaxation time, we explore the dynamics of SFQ-circuit-induced QPs. Our findings reveal that the QP density near the qubit reaches its peak after several microseconds of SFQ circuit operation, which corresponds to the phonon-mediated propagation time of QPs in the hybrid circuits. This suggests that phonon-mediated propagation dominates the spreading of QPs in the hybrid circuits. Our results lay the foundation to suppress QP poisoning in quantum-classical hybrid systems.
arxiv pdf