George A. Keefe

Tunable Superconducting Qubits with Flux-Independent Coherence

  1. M. D. Hutchings,
  2. Jared B. Hertzberg,
  3. Yebin Liu,
  4. Nicholas T. Bronn,
  5. George A. Keefe,
  6. Jerry M. Chow,
  7. and B. L. T. Plourde
We have studied the impact of low-frequency magnetic flux noise upon superconducting transmon qubits with various levels of tunability. We find that qubits with weaker tunability exhibit
dephasing that is less sensitive to flux noise. This insight was used to fabricate qubits where dephasing due to flux noise was suppressed below other dephasing sources, leading to flux-independent dephasing times T2* ~ 15 us over a tunable range of ~340 MHz. Such tunable qubits have the potential to create high-fidelity, fault-tolerant qubit gates and fundamentally improve scalability for a quantum processor.
arxiv pdf

Superconducting qubit in waveguide cavity with coherence time approaching 0.1ms

  1. Chad Rigetti,
  2. Stefano Poletto,
  3. Jay M. Gambetta,
  4. B. L. T. Plourde,
  5. Jerry M. Chow,
  6. A. D. Corcoles,
  7. John A. Smolin,
  8. Seth T. Merkel,
  9. J. R. Rozen,
  10. George A. Keefe,
  11. Mary B. Rothwell,
  12. Mark B. Ketchen,
  13. and M. Steffen
We report a superconducting artificial atom with an observed quantum coherence time of T2*=95us and energy relaxation time T1=70us. The system consists of a single Josephson junction
transmon qubit embedded in an otherwise empty copper waveguide cavity whose lowest eigenmode is dispersively coupled to the qubit transition. We attribute the factor of four increase in the coherence quality factor relative to previous reports to device modifications aimed at reducing qubit dephasing from residual cavity photons. This simple device holds great promise as a robust and easily produced artificial quantum system whose intrinsic coherence properties are sufficient to allow tests of quantum error correction.
arxiv pdf