Kuang Liu

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

Single-flux-quantum-based Qubit Control with Tunable Driving Strength

  1. Kuang Liu,
  2. Yifan Wang,
  3. Bo Ji,
  4. Wanpeng Gao,
  5. Zhirong Lin,
  6. and Zhen Wang
Single-flux-quantum (SFQ) circuits have great potential in building cryogenic quantum-classical interfaces for scaling up superconducting quantum processors. SFQ-based quantum gates
have been designed and realized. However, current control schemes are difficult to tune the driving strength to qubits, which restricts the gate length and usually induces leakage to unwanted levels. In this study, we design the scheme and corresponding pulse generator circuit to continuously adjust the driving strength by coupling SFQ pulses with variable intervals. This scheme not only provides a way to adjust the SFQ-based gate length, but also proposes the possibility to tune the driving strength envelope. Simulations show that our scheme can suppress leakage to unwanted levels and reduce the error of SFQ-based Clifford gates by more than an order of magnitude.
arxiv pdf