K. Rodbell

Quasiparticle tunneling as a probe of Josephson junction quality and capacitor material in superconducting qubits

  1. C. Kurter,
  2. C. E. Murray,
  3. R.T. Gordon,
  4. B. B. Wymore,
  5. M. Sandberg,
  6. R. M. Shelby,
  7. A. Eddins,
  8. V. P. Adiga,
  9. A. D. K. Finck,
  10. E. Rivera,
  11. A.A. Stabile,
  12. B. Trimm,
  13. B. Wacaser,
  14. K. Balakrishnan,
  15. A. Pyzyna,
  16. J. Sleight,
  17. M. Steffen,
  18. and K. Rodbell
Non-equilibrium quasiparticles are possible sources for decoherence in superconducting qubits because they can lead to energy decay or dephasing upon tunneling across Josephson junctions.
Here, we investigate the impact of the intrinsic properties of two-dimensional transmon qubits on quasiparticle tunneling (QPT) and discuss how we can use QPT to gain critical information about the Josephson junction quality and device performance. We find the tunneling rate of the non-equilibrium quasiparticles to be sensitive to the choice of the shunting capacitor material and their geometry in qubits. In some devices, we observe an anomalous temperature dependence of the QPT rate below 100 mK that deviates from a constant background associated with non-equilibrium quasiparticles. We speculate that high transmission sites within the Josephson junction’s tunnel barrier can lead to this behavior, which we can model by assuming that the defect sites have a smaller effective superconducting gap than the leads of the junction. Our results present a unique characterization tool for tunnel barrier quality in Josephson junctions and shed light on how quasiparticles can interact with various elements of the qubit circuit.
arxiv pdf

Environmental Radiation Impact on Lifetimes and Quasiparticle Tunneling Rates of Fixed-Frequency Transmon Qubits

  1. R.T. Gordon,
  2. C. E. Murray,
  3. C. Kurter,
  4. M. Sandberg,
  5. S.A. Hall,
  6. K. Balakrishnan,
  7. R. Shelby,
  8. B. Wacaser,
  9. A.A. Stabile,
  10. J.W. Sleight,
  11. M. Brink,
  12. M. B. Rothwell,
  13. K. Rodbell,
  14. O. Dial,
  15. and M. Steffen
Quantum computing relies on the operation of qubits in an environment as free of noise as possible. This work reports on measuring the impact of environmental radiation on lifetimes
of fixed frequency transmon qubits with various capacitor pad geometries by varying the amount of shielding used in the measurement space. It was found that the qubit lifetimes are robust against these shielding changes until the most extreme limit was tested without a mixing chamber shield in the refrigerator. In contrast, the quasiparticle tunneling rates were found to be extremely sensitive to all configurations tested, indicating these devices are not yet limited by losses related to superconducting quasiparticles.
arxiv pdf