Terry P. Orlando

Suppressing relaxation in superconducting qubits by quasiparticle pumping

  1. Simon Gustavsson,
  2. Fei Yan,
  3. Gianluigi Catelani,
  4. Jonas Bylander,
  5. Archana Kamal,
  6. Jeffrey Birenbaum,
  7. David Hover,
  8. Danna Rosenberg,
  9. Gabriel Samach,
  10. Adam P. Sears,
  11. Steven J. Weber,
  12. Jonilyn L. Yoder,
  13. John Clarke,
  14. Andrew J. Kerman,
  15. Fumiki Yoshihara,
  16. Yasunobu Nakamura,
  17. Terry P. Orlando,
  18. and William D. Oliver
Dynamical error suppression techniques are commonly used to improve coherence in quantum systems. They reduce dephasing errors by applying control pulses designed to reverse erroneous
coherent evolution driven by environmental noise. However, such methods cannot correct for irreversible processes such as energy relaxation. In this work, we investigate a complementary, stochastic approach to reducing errors: instead of deterministically reversing the unwanted qubit evolution, we use control pulses to shape the noise environment dynamically. In the context of superconducting qubits, we implement a pumping sequence to reduce the number of unpaired electrons (quasiparticles) in close proximity to the device. We report a 70% reduction in the quasiparticle density, resulting in a threefold enhancement in qubit relaxation times, and a comparable reduction in coherence variability.
arxiv pdf

Coherence and Decay of Higher Energy Levels of a Superconducting Transmon Qubit

  1. Michael J. Peterer,
  2. Samuel J. Bader,
  3. Xiaoyue Jin,
  4. Fei Yan,
  5. Archana Kamal,
  6. Ted Gudmundsen,
  7. Peter J. Leek,
  8. Terry P. Orlando,
  9. William D. Oliver,
  10. and Simon Gustavsson
We present measurements of coherence and successive decay dynamics of higher energy levels of a superconducting transmon qubit. By applying consecutive π-pulses for each sequential
transition frequency, we excite the qubit from the ground state up to its fourth excited level and characterize the decay and coherence of each state. We find the decay to proceed mainly sequentially, with relaxation times in excess of 20 μs for all transitions. We also provide a direct measurement of the charge dispersion of these levels by analyzing beating patterns in Ramsey fringes. The results demonstrate the feasibility of using higher levels in transmon qubits for encoding quantum information.
arxiv pdf

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