Wei-Ren Syong

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

An Effective Reflection Mode Measurement for Hanger-Coupled Microwave Resonators

  1. John R. Pitten,
  2. Nicholas Materise,
  3. Wei-Ren Syong,
  4. Jorge Ramirez,
  5. Douglas Bennett,
  6. and Corey Rae H McRae
Superconducting microwave resonators are used to study two-level system (TLS) loss in superconducting quantum devices. Fano asymmetry, characterized by a nonzero asymmetry angle ϕ
in the diameter correction method (DCM), results from the coupling schemes used to measure these devices, including the commonly used hanger method. ϕ is an additional fitting parameter which contains no physically interesting information and can obscure device parameters of interest. The tee-junction symmetry nominally present in these resonator devices provides an avenue for the elimination of Fano asymmetry using calibrated measurement. We show that the eigenvalue associated with the common mode excitation of the resonator is an effective reflection mode (ERM) which has no Fano asymmetry. Our analysis reveals the cause of Fano asymmetry as interference between common and differential modes. Practically, we obtain the ERM from a linear combination of calibrated reflection and transmission measurements. We utilize a 3D aluminum cavity to experimentally demonstrate the validity and flexibility of this model. To extend the usefulness of this symmetry analysis, we apply perturbation theory to recover the ERM in a multiplexed coplanar waveguide resonator device and experimentally demonstrate quantitative agreement in the extracted Q−1i between hanger mode and ERM measurements. We observe a five-fold reduction in uncertainty from the ERM compared to the standard hanger mode at the lowest measured power, -160 dBm delivered to the device. This method could facilitate an increase in throughput of low-power superconducting resonator measurements by up to a factor of 25, as well as allow the extraction of critical parameters from otherwise unfittable device data.
arxiv pdf