J. Sleight

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

Investigating surface loss effects in superconducting transmon qubits

  1. J. M. Gambetta,
  2. C. E. Murray,
  3. Y.-K.-K. Fung,
  4. D. T. McClure,
  5. O. Dial,
  6. W. Shanks,
  7. J. Sleight,
  8. and M. Steffen
Superconducting qubits are sensitive to a variety of loss mechanisms including dielectric loss from interfaces. By changing the physical footprint of the qubit it is possible to modulate
sensitivity to surface loss. Here we show a systematic study of planar superconducting transmons of differing physical footprints to optimize the qubit design for maximum coherence. We find that qubits with small footprints are limited by surface loss and that qubits with large footprints are limited by other loss mechanisms which are currently not understood.
arxiv pdf