Luming Duan

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

Quantum interface between a transmon qubit and spins of nitrogen-vacancy centers

  1. Yaowen Hu,
  2. Yipu Song,
  3. and Luming Duan
Hybrid quantum circuits combining advantages of each individual system have provided a promising platform for quantum information processing. Here we propose an experimental scheme
to directly couple a transmon qubit to an individual spin in the nitrogen-vacancy (NV) center, with a coupling strength three orders of magnitude larger than that for a single spin coupled to a microwave cavity. This direct coupling between the transmon and the NV center could be utilized to make a transmon bus, leading to a coherently virtual exchange among different single spins. Furthermore, we demonstrate that, by coupling a transmon to a low-density NV ensemble, a SWAP operation between the transmon and NV ensemble is feasible and a quantum non-demolition measurement on the state of NV ensemble can be realized on the cavity-transmon-NV-ensemble hybrid system. Moreover, on this system, a virtual coupling can be achieved between the cavity and NV ensemble, which is much larger in magnitude than the direct coupling between the cavity and the NV ensemble. The photon state in cavity can be thus stored into NV spins more efficiently through this virtual coupling.
arxiv pdf