A. Eddins

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

High-efficiency measurement of an artificial atom embedded in a parametric amplifier

  1. A. Eddins,
  2. J.M. Kreikebaum,
  3. D.M. Toyli,
  4. E.M. Levenson-Falk,
  5. A. Dove,
  6. W.P. Livingston,
  7. B.A. Levitan,
  8. L. C. G. Govia,
  9. A. A. Clerk,
  10. and I. Siddiqi
A crucial limit to measurement efficiencies of superconducting circuits comes from losses involved when coupling to an external quantum amplifier. Here, we realize a device circumventing
this problem by directly embedding a two-level artificial atom, comprised of a transmon qubit, within a flux-pumped Josephson parametric amplifier. Surprisingly, this configuration is able to enhance dispersive measurement without exposing the qubit to appreciable excess backaction. This is accomplished by engineering the circuit to permit high-power operation that reduces information loss to unmonitored channels associated with the amplification and squeezing of quantum noise. By mitigating the effects of off-chip losses downstream, the on-chip gain of this device produces end-to-end measurement efficiencies of up to 80 percent. Our theoretical model accurately describes the observed interplay of gain and measurement backaction, and delineates the parameter space for future improvement. The device is compatible with standard fabrication and measurement techniques, and thus provides a route for definitive investigations of fundamental quantum effects and quantum control protocols.
arxiv pdf