C. Kurter

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

Suppressed crosstalk between two-junction superconducting qubits with mode-selective exchange coupling

  1. A. D. K. Finck,
  2. S. Carnevale,
  3. D. Klaus,
  4. C. Scerbo,
  5. J. Blair,
  6. T.G. McConkey,
  7. C. Kurter,
  8. A. Carniol,
  9. G. Keefe,
  10. M. Kumph,
  11. and O. E. Dial
Fixed-frequency qubits can suffer from always-on interactions that inhibit independent control. Here, we address this issue by experimentally demonstrating a superconducting architecture
using qubits that comprise of two capacitively-shunted Josephson junctions connected in series. Historically known as tunable coupling qubits (TCQs), such two-junction qubits support two modes with distinct frequencies and spatial symmetries. By selectively coupling only one type of mode and using the other as our computational basis, we greatly suppress crosstalk between the data modes while permitting all-microwave two-qubit gates.
arxiv pdf

Merged-Element Transmons: Design and Qubit Performance

  1. H. J. Mamin,
  2. E. Huang,
  3. S. Carnevale,
  4. C. T. Rettner,
  5. N. Arellano,
  6. M. H. Sherwood,
  7. C. Kurter,
  8. B. Trimm,
  9. M. Sandberg,
  10. R. M. Shelby,
  11. M. A. Mueed,
  12. B. A. Madon,
  13. A. Pushp,
  14. M. Steffen,
  15. and D. Rugar
We have demonstrated a novel type of superconducting transmon qubit in which a Josephson junction has been engineered to act as its own parallel shunt capacitor. This merged-element
transmon (MET) potentially offers a smaller footprint and simpler fabrication than conventional transmons. Because it concentrates the electromagnetic energy inside the junction, it reduces relative electric field participation from other interfaces. By combining micrometer-scale Al/AlOx/Al junctions with long oxidations and novel processing, we have produced functional devices with EJ/EC in the low transmon regime (EJ/EC ≲30). Cryogenic I-V measurements show sharp dI/dV structure with low sub-gap conduction. Qubit spectroscopy of tunable versions show a small number of avoided level crossings, suggesting the presence of two-level systems (TLS). We have observed mean T1 times typically in the range of 10-90 microseconds, with some annealed devices exhibiting T1 > 100 microseconds over several hours. The results suggest that energy relaxation in conventional, small-junction transmons is not limited by junction loss.
arxiv pdf