Halldór Jakobsson

Suppressing microwave crosstalk in superconducting quantum processors by designing parasitic drive capacitances

  1. Anuj Aggarwal,
  2. Jorge Fernández-Pendás,
  3. Simon Pettersson Fors,
  4. Zakaria Mohamed,
  5. Halldór Jakobsson,
  6. Amr Osman,
  7. Janka Biznárová,
  8. Andreas Nylander,
  9. Anita Fadavi Roudsari,
  10. Marcus Rommel,
  11. Anton Frisk Kockum,
  12. and Giovanna Tancredi
As superconducting quantum processors scale to large qubit counts, unwanted microwave drive crosstalk becomes an important source of coherent errors. Here, we investigate the role of
parasitic coupling paths in the suppression of drive crosstalk in a processor with two fixed-frequency transmon qubits and a tunable coupler. To account for the many different coupling paths, we develop a detailed circuit model to capture the dependence of crosstalk on the coupler frequency. We experimentally demonstrate suppression of crosstalk, finding that the suppression does not originate solely from cancellation of the effective qubit-qubit coupling, but also is a result of interference between other coupling paths. Building on these insights, we show that drive crosstalk can be suppressed with the right choice of parasitic capacitances, which otherwise often are treated as unwanted design imperfections. Our results provide practical guidance for low-crosstalk device design of superconducting quantum processors and point toward a broader strategy in which parasitic coupling paths are engineered rather than simply minimized.
arxiv pdf

Electrical post-fabrication tuning of aluminum Josephson junctions at room temperature

  1. Christian Križan,
  2. Maurizio Toselli,
  3. Irshad Ahmad,
  4. Hadi Khaksaran,
  5. Marcus Rommel,
  6. Nermin Trnjanin,
  7. Janka Biznárová,
  8. Mamta Dahiya,
  9. Emil Hogedal,
  10. Halldór Jakobsson,
  11. Andreas Nylander,
  12. Jonas Bylander,
  13. Per Delsing,
  14. and Giovanna Tancredi
Josephson junctions are a key element of superconducting quantum technology, serving as the core building blocks of superconducting qubits. We present an experimental study on room-temperature
electrical tuning of aluminum junctions, showing that voltage pulses can controllably increase their resistance and adjust the Josephson energy while maintaining qubit quality factors above 1 million. We find that the rate of resistance increase scales exponentially with pulse amplitude during manipulation, after which the spontaneous resistance increase scales proportionally to the amount of manipulation. We show that this spontaneous increase halts at cryogenic temperatures, and resumes again at room temperature. Using our stepwise protocol, we achieve up to a 270% increase in junction resistance, corresponding to a reduction of nearly 2 GHz of the qubit transition frequency. These results establish the achievable range, relaxation behavior, and practical limits of electrical tuning, enabling post-fabrication mitigation of frequency crowding in quantum processors.
arxiv pdf

Mitigating transients in flux-control signals in a superconducting quantum processor

  1. Anuj Aggarwal,
  2. Jorge Fernández-Pendás,
  3. Tahereh Abad,
  4. Daryoush Shiri,
  5. Halldór Jakobsson,
  6. Marcus Rommel,
  7. Andreas Nylander,
  8. Emil Hogedal,
  9. Amr Osman,
  10. Janka Biznárová,
  11. Robert Rehammar,
  12. Michele Faucci Giannelli,
  13. Anita Fadavi Roudsari,
  14. Jonas Bylander,
  15. and Giovanna Tancredi
Flux-tunable qubits and couplers are common components in superconducting quantum processors. However, dynamically controlling these elements via current pulses poses challenges due
to distortions and transients in the propagating signals. In particular, long-time transients can persist, adversely affecting subsequent qubit control operations. We model the flux control line as a first-order RC circuit and introduce a class of pulses designed to mitigate long-time transients. We theoretically demonstrate the robustness of these pulses against parameter mischaracterization and provide experimental evidence of their effectiveness in mitigating transients when applied to a flux-tunable qubit coupler. The proposed pulse design offers a practical solution for mitigating long-time transients, enabling efficient and reliable experiment tune-ups without requiring detailed flux line characterization.
arxiv pdf