Joel N. Ullom

Precise 2D electric field density simulations for superconducting quantum devices

  1. Zydrunas Gimbutas,
  2. Wei-Ren Syong,
  3. Nhi Nguyen,
  4. Ariana Taylor,
  5. Joel N. Ullom,
  6. Bradley K. Alpert,
  7. Douglas A. Bennett,
  8. and Corey Rae H McRae
Dielectric loss due to two-level systems is a limiting factor for superconducting qubit relaxation times. These losses arise mostly from nanometer-scale interfacial defect regions in
superconducting devices with planar dimensions of microns to millimeters, thus making it resource intensive to accurately simulate the electric field density in these regions with traditional electromagnetic solvers. In this work, we demonstrate a fast boundary integral equation solver that allows precise simulation of electric field density in these thin regions, showing a speedup of around two orders of magnitude over traditional solvers, with relative errors around 10−7 for a ten-minute solution runtime. By computing participation ratios through Green’s first identity without squaring the electric field, our approach is less susceptible to the field singularities near conductor corners. We apply this solver to a basic untrenched coplanar waveguide cross-section, showing that the common assumption of participation ratio linearity with dielectric constant holds well for some interfaces and not others; in particular, while the metal-air (MA) top and corner follow this linear relationship strongly, the MA sidewall does not. We then compare isotropic and anisotropic etching, showing that the MA sidewall and the metal-air-substrate triple junction are the most strongly affected. We are currently leveraging this solver to explore geometries that will uniquely isolate the participation ratios of the different dielectrics. Finally, we are working to combine this solver framework with a full 3D microwave solver to accurately calculate participation ratios for the thin dielectrics that are known sources of loss in superconducting qubits.
arxiv pdf

Spectroscopic measurements and models of energy deposition in the substrate of quantum circuits by natural ionizing radiation

  1. Joseph W. Fowler,
  2. Paul Szypryt,
  3. Raymond Bunker,
  4. Ellen R. Edwards,
  5. Ian Fogarty Florang,
  6. Jiansong Gao,
  7. Andrea Giachero,
  8. Shannon F. Hoogerheide,
  9. Ben Loer,
  10. H. Pieter Mumm,
  11. Nathan Nakamura,
  12. Galen C. O'Neil,
  13. John L. Orrell,
  14. Elizabeth M. Scott,
  15. Jason Stevens,
  16. Daniel S. Swetz,
  17. Brent A. VanDevender,
  18. Michael Vissers,
  19. and Joel N. Ullom
Naturally occurring background radiation is a source of correlated decoherence events in superconducting qubits that will challenge error-correction schemes. To characterize the radiationenvironment in an unshielded laboratory, we performed broadband, spectroscopic measurements of background events in silicon substrates located inside a millikelvin refrigerator, an environment representative of superconducting qubit systems. We measured the background spectra in silicon substrates of two thicknesses, 0.5 mm and 1.5 mm, and obtained the average event rate and the integrated power deposition. In a 25 mm^2 area and the thinner substrate, these values are 0.023 events per second and 4.9 keV/s, counting events that deposit at least 40 keV. We find the background spectrum to be nearly featureless. Its intensity decreases by a factor of 40,000 between 100 keV and 3 MeV for silicon substrates 0.5 mm thick. We find the cryogenic measurements to be in good agreement with predictions based on measurements of the terrestrial gamma-ray flux, published models of cosmic-ray fluxes, a crude model of the cryostat, and radiation-transport simulations. No free parameters are required to predict the background spectra in the silicon substrates. The good agreement between measurements and predictions allow assessment of the relative contributions of terrestrial and cosmic background sources and their dependence on substrate thickness. Our spectroscopic measurements are performed with superconducting microresonators that transduce deposited energy to a readily detectable electrical signal. We find that gamma-ray emissions from radioisotopes are responsible for the majority of events depositing E<1.5 MeV, while nucleons among the cosmic-ray secondary particles cause most events that deposit more energy. These results suggest several paths to reducing the impact of background radiation on quantum circuits.[/expand]
arxiv pdf