Kate Azar

Dephasing in Fluxonium Qubits from Coherent Quantum Phase Slips

  1. Mallika T. Randeria,
  2. Thomas M. Hazard,
  3. Agustin Di Paolo,
  4. Kate Azar,
  5. Max Hays,
  6. Leon Ding,
  7. Junyoung An,
  8. Michael Gingras,
  9. Bethany M. Niedzielski,
  10. Hannah Stickler,
  11. Jeffrey A. Grover,
  12. Jonilyn L. Yoder,
  13. Mollie E. Schwartz,
  14. William D. Oliver,
  15. and Kyle Serniak
Phase slips occur across all Josephson junctions (JJs) at a rate that increases with the impedance of the junction. In superconducting qubits composed of JJ-array superinductors —
such as fluxonium — phase slips in the array can lead to decoherence. In particular, phase-slip processes at the individual array junctions can coherently interfere, each with an Aharonov–Casher phase that depends on the offset charges of the array islands. These coherent quantum phase slips (CQPS) perturbatively modify the qubit frequency, and therefore charge noise on the array islands will lead to dephasing. By varying the impedance of the array junctions, we design a set of fluxonium qubits in which the expected phase-slip rate within the JJ-array changes by several orders of magnitude. We characterize the coherence times of these qubits and demonstrate that the scaling of CQPS-induced dephasing rates agrees with our theoretical model. Furthermore, we perform noise spectroscopy of two qubits in regimes dominated by either CQPS or flux noise. We find the noise power spectrum associated with CQPS dephasing appears to be featureless at low frequencies and not 1/f. Numerical simulations indicate this behavior is consistent with charge noise generated by charge-parity fluctuations within the array. Our findings broadly inform JJ-array-design tradeoffs, relevant for the numerous superconducting qubit designs employing JJ-array superinductors.
arxiv pdf

High-Fidelity, Frequency-Flexible Two-Qubit Fluxonium Gates with a Transmon Coupler

  1. Leon Ding,
  2. Max Hays,
  3. Youngkyu Sung,
  4. Bharath Kannan,
  5. Junyoung An,
  6. Agustin Di Paolo,
  7. Amir H. Karamlou,
  8. Thomas M. Hazard,
  9. Kate Azar,
  10. David K. Kim,
  11. Bethany M. Niedzielski,
  12. Alexander Melville,
  13. Mollie E. Schwartz,
  14. Jonilyn L. Yoder,
  15. Terry P. Orlando,
  16. Simon Gustavsson,
  17. Jeffrey A. Grover,
  18. Kyle Serniak,
  19. and William D. Oliver
We propose and demonstrate an architecture for fluxonium-fluxonium two-qubit gates mediated by transmon couplers (FTF, for fluxonium-transmon-fluxonium). Relative to architectures that
exclusively rely on a direct coupling between fluxonium qubits, FTF enables stronger couplings for gates using non-computational states while simultaneously suppressing the static controlled-phase entangling rate (ZZ) down to kHz levels, all without requiring strict parameter matching. Here we implement FTF with a flux-tunable transmon coupler and demonstrate a microwave-activated controlled-Z (CZ) gate whose operation frequency can be tuned over a 2 GHz range, adding frequency allocation freedom for FTF’s in larger systems. Across this range, state-of-the-art CZ gate fidelities were observed over many bias points and reproduced across the two devices characterized in this work. After optimizing both the operation frequency and the gate duration, we achieved peak CZ fidelities in the 99.85-99.9\% range. Finally, we implemented model-free reinforcement learning of the pulse parameters to boost the mean gate fidelity up to 99.922±0.009%, averaged over roughly an hour between scheduled training runs. Beyond the microwave-activated CZ gate we present here, FTF can be applied to a variety of other fluxonium gate schemes to improve gate fidelities and passively reduce unwanted ZZ interactions.
arxiv pdf