Nhi Nguyen

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

Low-loss Nb on Si superconducting resonators from a dual-use spintronics deposition chamber and with acid-free post-processing

  1. Maciej W. Olszewski,
  2. Jadrien T. Paustian,
  3. Tathagata Banerjee,
  4. Haoran Lu,
  5. Jorge L. Ramirez,
  6. Nhi Nguyen,
  7. Kiichi Okubo,
  8. Rohit Pant,
  9. Aleksandra B. Biedron,
  10. Daniel C. Ralph,
  11. Christopher J. K. Richardson,
  12. Gregory D. Fuchs,
  13. Corey Rae H McRae,
  14. Ivan V. Pechenezhskiy,
  15. B. L. T. Plourde,
  16. and Valla Fatemi
Magnetic impurities are known to degrade superconductivity. For this reason, physical vapor deposition chambers that have previously been used for magnetic materials have generally
been avoided for making high-quality superconducting resonator devices. In this article, we show by example that such chambers can be used: with Nb films sputtered in a chamber that continues to be used for magnetic materials, we demonstrate compact (3 {\mu}m gap) coplanar waveguide resonators with low-power internal quality factors near one million. We achieve this using a resist strip bath with no post-fabrication acid treatment, which results in performance comparable to previous strip baths with acid treatments. We also find evidence that this improved resist strip bath provides a better surface chemical template for post-fabrication hydrogen fluoride processing. These results are consistent across three Si substrate preparation methods, including a \SI{700}{\celsius} anneal.
arxiv pdf