T. Cai

Elimination of unwanted qubit interactions for parametric exchange two-qubit gates in a tunable coupling circuit

  1. X. Han,
  2. T. Cai,
  3. X. Li,
  4. Y. Wu,
  5. Y. Ma,
  6. J. Wang,
  7. H. Zhang,
  8. Yipu Song,
  9. and Luming Duan
We experimentally demonstrate a simple-design tunable coupler, achieving a continuous tunability for eliminating unwanted qubit interactions. We implement two-qubit iSWAP gate by applying
a fast-flux bias modulation pulse on the coupler to turn on parametric exchange interaction between computational qubits. Aiming to fully investigate error sources on the two-qubit gates, we perform quantum process tomography measurements and numerical simulations as varying static ZZ coupling strength. Our results reveal that the change in the two-qubit gate error is mainly attributed to unwanted high-frequency oscillation error terms, while the dynamic ZZ coupling parasitising in two-qubit gate operation may also contribute to the dependency of the gate fidelity. This approach, which has not yet been previously explored, provides a guiding principle to improve gate fidelity of parametric iSWAP gate by the elimination of unwanted qubit interactions. This controllable interaction, together with the parametric architecture by using modulation techniques, is desirable for crosstalk free multiqubit quantum circuits and quantum simulation applications.
arxiv pdf

A tunable coupler for suppressing adjacent superconducting qubit coupling

  1. X. Li,
  2. T. Cai,
  3. H. Yan,
  4. Z. Wang,
  5. X. Pan,
  6. Y. Ma,
  7. W. Cai,
  8. J. Han,
  9. Z. Hua,
  10. X. Han,
  11. Y. Wu,
  12. H. Zhang,
  13. H. Wang,
  14. Yipu Song,
  15. Luming Duan,
  16. and Luyan Sun
Controllable interaction between superconducting qubits is desirable for large-scale quantum computation and simulation. Here, based on a theoretical proposal by Yan et al. [Phys. Rev.
Appl. 10, 054061 (2018)] we experimentally demonstrate a simply-designed and flux-controlled tunable coupler with continuous tunability by adjusting the coupler frequency, which can completely turn off adjacent superconducting qubit coupling. Utilizing the tunable interaction between two qubits via the coupler, we implement a controlled-phase (CZ) gate by tuning one qubit frequency into and out of the usual operating point while dynamically keeping the qubit-qubit coupling off. This scheme not only efficiently suppresses the leakage out of the computational subspace but also allows for the acquired two-qubit phase being geometric at the operating point only where the coupling is on. We achieve an average CZ gate fidelity of 98.3%, which is dominantly limited by qubit decoherence. The demonstrated tunable coupler provides a desirable tool to suppress adjacent qubit coupling and is suitable for large-scale quantum computation and simulation.
arxiv pdf