Lukas Pahl

Bosonic Error Correction with Fluxonium

  1. Shantanu R. Jha,
  2. Shoumik D. Chowdhury,
  3. Gabriele Rolleri,
  4. Anaida Ali,
  5. Lev-Arcady Sellem,
  6. Réouven Assouly,
  7. David Pahl,
  8. Lukas Pahl,
  9. Junyoung An,
  10. Farid Hassani,
  11. Hung-Yu Tsao,
  12. Chia-Chin Tsai,
  13. Aranya Goswami,
  14. Jeremie Boudreault,
  15. Jeffrey M. Gertler,
  16. Michael A. Gingras,
  17. Bethany M. Niedzielski,
  18. Jeffrey M. Knecht,
  19. Mollie E. Schwartz,
  20. Kyle Serniak,
  21. Jeffrey A. Grover,
  22. Baptiste Royer,
  23. Max Hays,
  24. and William D. Oliver
Bosonic quantum error correction (QEC) offers a hardware-efficient route to fault-tolerant quantum computing. To date, however, superconducting circuit implementations of bosonic codes
have utilized centimeter-scale 3D microwave cavities controlled by fixed-frequency transmon qubits, with logical lifetimes limited by transmon bit-flip errors. Here, we realize bosonic QEC in a fully planar architecture by pairing a heavy fluxonium, whose 451±70 μs bit-flip lifetime exceeds that of any control qubit in previous demonstrations, with an on-chip Archimedean spiral resonator several orders of magnitude smaller in mode volume than prior 3D cavities. We prepare finite-energy Gottesman-Kitaev-Preskill (GKP) states and stabilize them using measurement-free error correction with rapid fluxonium reset, extending the logical lifetime by a factor of 1.59±0.05. These results provide the first demonstration of resonator control using a weakly coupled fluxonium and, with it, the first realization of standalone bosonic QEC in a fully planar superconducting circuit architecture.
arxiv pdf

Adaptive Spectroscopy of Fast Two-Level-System Dynamics in Superconducting Qubits

  1. Fabrizio Berritta,
  2. David Pahl,
  3. Lukas Pahl,
  4. William P. Banner,
  5. Gabriel Cutter,
  6. Jan A. Krzywda,
  7. Spencer Weeden,
  8. Shravan Patel,
  9. Paul Buttles,
  10. Stanislav Eilhart,
  11. Michael Gingras,
  12. Bethany M. Niedzielski,
  13. Robert McDermott,
  14. Mollie E. Schwartz,
  15. Kyle Serniak,
  16. Max Hays,
  17. Jeffrey A. Grover,
  18. and William D. Oliver
Parasitic two-level-system (TLS) defects are a major source of energy relaxation and temporal instability in superconducting quantum processors. Our sub-second adaptive spectroscopy
reveals telegraphic switching of TLSs with a characteristic timescale of a few seconds and spectral diffusion with diffusivity D≈0.9 MHz2/s. These timescales are about 3×102 times faster than what is observed in conventional nonadaptive spectroscopy, which typically requires hours of measurement time. We resolve such fast dynamics on a field-programmable gate array (FPGA)-based controller that enables measurement of frequency- and time-resolved relaxations with sub-second temporal resolution in flux-tunable superconducting qubits. We observe similar defect dynamics across multiple qubits in independently fabricated devices measured in different laboratories. We correlate TLS-induced fluctuations with gate-level errors using randomized benchmarking. Our results reveal a previously inaccessible regime of frequency-resolved TLS dynamics and redefine the timescales relevant to TLS-aware characterization and calibration of superconducting quantum processors.
arxiv pdf

Placing and Routing Non-Local Quantum Error Correcting Codes in Multi-Layer Superconducting Qubit Hardware

  1. Melvin Mathews,
  2. Lukas Pahl,
  3. David Pahl,
  4. Vaishnavi L. Addala,
  5. Catherine Tang,
  6. William D. Oliver,
  7. and Jeffrey A. Grover
Quantum error correcting codes (QECCs) with asymptotically lower overheads than the surface code require non-local connectivity. Leveraging multi-layer routing and long-range coupling
capabilities in superconducting qubit hardware, we develop Hardware-Aware Layout, HAL: a robust, runtime-efficient heuristic algorithm that automates and optimizes the placement and routing of arbitrary QECCs. Using HAL, we perform a comparative study of hardware cost across various families of QECCs, including the bivariate bicycle codes, the open-boundary tile codes, and the constant-depth-decodable radial codes. The layouts produced by HAL confirm that open boundaries significantly reduce the hardware cost, while incurring reductions in logical efficiency. Among the best-performing codes were low-weight radial codes, despite lacking topological structure. Overall, HAL provides a valuable framework for evaluating the hardware feasibility of existing QECCs and guiding the discovery of new codes compatible with realistic hardware constraints.
arxiv pdf

Efficient Qubit Calibration by Binary-Search Hamiltonian Tracking

  1. Fabrizio Berritta,
  2. Jacob Benestad,
  3. Lukas Pahl,
  4. Melvin Mathews,
  5. Jan A. Krzywda,
  6. Réouven Assouly,
  7. Youngkyu Sung,
  8. David K. Kim,
  9. Bethany M. Niedzielski,
  10. Kyle Serniak,
  11. Mollie E. Schwartz,
  12. Jonilyn L. Yoder,
  13. Anasua Chatterjee,
  14. Jeffrey A. Grover,
  15. Jeroen Danon,
  16. William D. Oliver,
  17. and Ferdinand Kuemmeth
We present a real-time method for calibrating the frequency of a resonantly driven qubit. The real-time processing capabilities of a controller dynamically compute adaptive probing
sequences for qubit-frequency estimation. Each probing time and drive frequency are calculated to divide the prior probability distribution into two branches, following a locally optimal strategy that mimics a conventional binary search. We show the algorithm’s efficacy by stabilizing a flux-tunable transmon qubit, leading to improved coherence and gate fidelity. By feeding forward the updated qubit frequency, the FPGA-powered control electronics also mitigates non-Markovian noise in the system, which is detrimental to quantum error correction. Our protocol highlights the importance of feedback in improving the calibration and stability of qubits subject to drift and can be readily applied to other qubit platforms.
arxiv pdf