Jianshe Liu

Qubit Energy Tuner Based on Single Flux Quantum Circuits

  1. Xiao Geng,
  2. Rutian Huang,
  3. Yongcheng He,
  4. Kaiyong He,
  5. Genting Dai,
  6. Liangliang Yang,
  7. Xinyu Wu,
  8. Qing Yu,
  9. Mingjun Cheng,
  10. Guodong Chen,
  11. Jianshe Liu,
  12. and Wei Chen
A device called qubit energy tuner (QET) based on single flux quantum (SFQ) circuits is proposed for Z control of superconducting qubits. Created from the improvement of flux digital-to-analog
converters (flux DACs), a QET is able to set the energy levels or the frequencies of qubits, especially flux-tunable transmons, and perform gate operations requiring Z control. The circuit structure of QET is elucidated, which consists of an inductor loop and flux bias units for coarse tuning or fine tuning. The key feature of a QET is analyzed to understand how SFQ pulses change the inductor loop current, which provides external flux for qubits. To verify the functionality of the QET, three simulations are carried out. The first one verifies the responses of the inductor loop current to SFQ pulses. The results show that there is about 4.2% relative deviation between analytical solutions of the inductor loop current and the solutions from WRSpice time-domain simulation. The second and the third simulations with QuTip show how a Z gate and an iSWAP gate can be performed by this QET, respectively, with corresponding fidelities 99.99884% and 99.93906% for only once gate operation to specific initial states. These simulations indicate that the SFQ-based QET could act as an efficient component of SFQ-based quantum-classical interfaces for digital Z control of large-scale superconducting quantum computers.
arxiv pdf

Quantum coherence between cavity and artificial atom in a superconducting circuit QED ladder system

  1. Qichun Liu,
  2. Han Cai,
  3. Yingshan Zhang,
  4. Jianshe Liu,
  5. and Wei Chen
We have created a quantum three-level ladder system with the cavity dispersive energy level in a superconducting circuit quantum electrodynamics system consisting of a transmon qubit
and a cavity, and have directly observed the Autler-Townes splitting eff?ect instead of representing it by the probability of the qubit being at each level. A coupler tone is applied on the transition between the second excited state of transmon and cavity dispersive level, while the cavity spectrum is probed. A doublet transmission and anormalous dispersion spectrum of the cavity level is clearly shown. The inverse Fourier transform of cavity spectrum indicates that there is a quantum coherence Rabi oscillation of the populations between cavity and qubit.
arxiv pdf

Charge-SQUID and Tunable Phase-slip Flux Qubit

  1. Hu Zhao,
  2. Tiefu Li,
  3. Jianshe Liu,
  4. Wei Chen,
  5. and J. Q. You
A phase-slip flux qubit, exactly dual to a charge qubit, is composed of a superconducting loop interrupted by a phase-slip junction. Here we propose a tunable phase-slip flux qubit
by replacing the phase-slip junction with a charge-related superconducting quantum interference device (SQUID) consisting of two phase-slip junctions connected in series with a superconducting island. This charge-SQUID acts as an effective phase-slip junction controlled by the applied gate voltage and can be used to tune the energy-level splitting of the qubit. Also, we show that a large inductance inserted in the loop can reduce the inductance energy and consequently suppress the dominating flux noise of the phase-slip flux qubit. This enhanced phase-slip flux qubit is exactly dual to a transmon qubit.
arxiv pdf