Yasunobu Nakamura

Improving quantum gate fidelities by using a qubit to measure microwave pulse distortions

  1. Simon Gustavsson,
  2. Olger Zwier,
  3. Jonas Bylander,
  4. Fei Yan,
  5. Fumiki Yoshihara,
  6. Yasunobu Nakamura,
  7. Terry P. Orlando,
  8. and William D. Oliver
We present a new method for determining pulse imperfections and improving the single-gate fidelity in a superconducting qubit. By applying consecutive positive and negative $pi$ pulses,
we amplify the qubit evolution due to microwave pulse distortion, which causes the qubit state to rotate around an axis perpendicular to the intended rotation axis. Measuring these rotations as a function of pulse period allows us to reconstruct the shape of the microwave pulse arriving at the sample. Using the extracted response to predistort the input signal, we are able to improve the pulse shapes and to reach an average single-qubit gate fidelity higher than 99.8%.
arxiv pdf

Dynamical decoupling and dephasing in interacting two-level systems

  1. Simon Gustavsson,
  2. Fei Yan,
  3. Jonas Bylander,
  4. Fumiki Yoshihara,
  5. Yasunobu Nakamura,
  6. Terry P. Orlando,
  7. and William D. Oliver
We implement dynamical decoupling techniques to mitigate noise and enhance the lifetime of an entangled state that is formed in a superconducting flux qubit coupled to a microscopic
two-level system. By rapidly changing the qubit’s transition frequency relative to the two-level system, we realize a refocusing pulse that reduces dephasing due to fluctuations in the transition frequencies, thereby improving the coherence time of the entangled state. The coupling coherence is further enhanced when applying multiple refocusing pulses, in agreement with our $1/f$ noise model. The results are applicable to any two-qubit system with transverse coupling, and they highlight the potential of decoupling techniques for improving two-qubit gate fidelities, an essential prerequisite for implementing fault-tolerant quantum computing.
arxiv pdf