Irshad Ahmad

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

Real-time adaptive tracking of fluctuating relaxation rates in superconducting qubits

  1. Fabrizio Berritta,
  2. Jacob Benestad,
  3. Jan A. Krzywda,
  4. Oswin Krause,
  5. Malthe A. Marciniak,
  6. Svend Krøjer,
  7. Christopher W. Warren,
  8. Emil Hogedal,
  9. Andreas Nylander,
  10. Irshad Ahmad,
  11. Amr Osman,
  12. Janka Biznárová,
  13. Marcus Rommel,
  14. Anita Fadavi Roudsari,
  15. Jonas Bylander,
  16. Giovanna Tancredi,
  17. Jeroen Danon,
  18. Jacob Hastrup,
  19. Ferdinand Kuemmeth,
  20. and Morten Kjaergaard
The fidelity of operations on a solid-state quantum processor is ultimately bounded by decoherence effects induced by a fluctuating environment. Characterizing environmental fluctuations
is challenging because the acquisition time of experimental protocols limits the precision with which the environment can be measured and may obscure the detailed structure of these fluctuations. Here we present a real-time Bayesian method for estimating the relaxation rate of a qubit, leveraging a classical controller with an integrated field-programmable gate array (FPGA). Using our FPGA-powered Bayesian method, we adaptively and continuously track the relaxation-time fluctuations of two fixed-frequency superconducting transmon qubits, which exhibit average relaxation times of approximately 0.17 ms and occasionally exceed 0.5 ms. Our technique allows for the estimation of these relaxation times in a few milliseconds, more than two orders of magnitude faster than previous nonadaptive methods, and allows us to observe fluctuations up to 5 times the qubit’s average relaxation rates on significantly shorter timescales than previously reported. Our statistical analysis reveals that these fluctuations occur on much faster timescales than previously understood, with two-level-system switching rates reaching up to 10 Hz. Our work offers an appealing solution for rapid relaxation-rate characterization in device screening and for improved understanding of fast relaxation dynamics.
arxiv pdf