Flexible Readout and Unconditional Reset for Superconducting Multi-Qubit Processors with Tunable Purcell Filters

  1. Yong-Xi Xiao,
  2. Da'er Feng,
  3. Xu-Yang Gu,
  4. Gui-Han Liang,
  5. Ming-Chuan Wang,
  6. Zheng-Yu Peng,
  7. Bing-Jie Chen,
  8. Yu Yan,
  9. Zheng-Yang Mei,
  10. Si-Lu Zhao,
  11. Yi-Zhou Bu,
  12. Cheng-Lin Deng,
  13. Xiaohui Song,
  14. Dongning Zheng,
  15. Yu-Xiang Zhang,
  16. Yun-Hao Shi,
  17. Zhongcheng Xiang,
  18. Kai Xu,
  19. and Heng Fan
Qubit readout and reset are critical components for the practical realization of quantum computing systems, as outlined by the DiVincenzo criteria. Here, we present a scalable architecture
employing frequency-tunable nonlinear Purcell filters designed specifically for superconducting qubits. This architecture enables flexible readout and unconditional reset functionalities. Our readout protocol dynamically adjusts the effective linewidth of the readout resonator through a tunable filter, optimizing the signal-to-noise ratio during measurement while suppressing photon noise during idle periods. Achieving a readout fidelity of 99.3% without using Josephson parametric amplifiers or traveling-wave parametric amplifiers, even with a small dispersive shift, demonstrates its effectiveness. For reset operations, our protocol utilizes the tunable coupler adjacent to the target qubit as an intermediary to channel qubit excitations into the Purcell filter, enabling rapid dissipation. We demonstrate unconditional reset of both leakage-induced |2⟩ and |1⟩ states within 200 ns (error rate ≤1%), and reset of the |1⟩ state alone in just 75 ns. Repeated reset cycles (≤600 ns) further reduce the error rate below 0.1%. Furthermore, the filter suppresses both photon noise and the Purcell effect, thereby reducing qubit decoherence. This scalable Purcell filter architecture shows exceptional performance in qubit readout, reset, and protection, marking it as a promising hardware component for advancing fault-tolerant quantum computing systems.

Stable and Efficient Charging of Superconducting C-shunt Flux Quantum Batteries

  1. Li Li,
  2. Si-Lu Zhao,
  3. Yun-Hao Shi,
  4. Bing-Jie Chen,
  5. Xinhui Ruan,
  6. Gui-Han Liang,
  7. Wei-Ping Yuan,
  8. Jia-Cheng Song,
  9. Cheng-Lin Deng,
  10. Yu Liu,
  11. Tian-Ming Li,
  12. Zheng-He Liu,
  13. Xue-Yi Guo,
  14. Xiaohui Song,
  15. Kai Xu,
  16. Heng Fan,
  17. Zhongcheng Xiang,
  18. and Dongning Zheng
Quantum batteries, as miniature energy storage devices, have sparked significant research interest in recent years. However, achieving rapid and stable energy transfer in quantum batteries
while obeying quantum speed limits remains a critical challenge. In this work, we experimentally optimize the charging process by leveraging the unique energy level structure of a superconducting capacitively-shunted flux qubit, using counterdiabatic pulses in the stimulated Raman adiabatic passage. Compared to previous studies, we impose two different norm constraints on the driving Hamiltonian, achieving optimal charging without exceeding the overall driving strength. Furthermore, we experimentally demonstrate a charging process that achieves the quantum speed limit. In addition, we introduce a dimensionless parameter  to unify charging speed and stability, offering a universal metric for performance optimization. In contrast to metrics such as charging power and thermodynamic efficiency, the  criterion quantitatively captures the stability of ergentropy while also considering the charging speed. Our results highlight the potential of the capacitively-shunted qubit platform as an ideal candidate for realizing three-level quantum batteries and deliver novel strategies for optimizing energy transfer protocols.