Bogdan C. Donose

Near-field localization of the boson peak on tantalum films for superconducting quantum devices

  1. Xiao Guo,
  2. Zachary Degnan,
  3. Julian Steele,
  4. Eduardo Solano,
  5. Bogdan C. Donose,
  6. Karl Bertling,
  7. Arkady Fedorov,
  8. Aleksandar D. Rakić,
  9. and Peter Jacobson
Superconducting circuits are among the most advanced quantum computing technologies, however their performance is currently limited by losses found in surface oxides and disordered
materials. Here, we identify and spatially localize a near-field signature of loss centers on tantalum films using terahertz scattering-type scanning near-field optical microscopy (s-SNOM). Making use of terahertz nanospectroscopy, we observe a localized excess vibrational mode around 0.5 THz and identify this resonance as the boson peak, a signature of amorphous materials. Grazing-incidence wide-angle x-ray scattering (GIWAXS) shows that oxides on freshly solvent-cleaned samples are amorphous, whereas crystalline phases emerge after aging in air. By localizing defect centers at the nanoscale, our characterization techniques and results will inform the optimization of fabrication procedures for new low-loss superconducting circuits.
arxiv pdf

Near-Field Terahertz Nanoscopy of Coplanar Microwave Resonators

  1. Xiao Guo,
  2. Xin He,
  3. Zach Degnan,
  4. Bogdan C. Donose,
  5. Karl Bertling,
  6. Arkady Fedorov,
  7. Aleksandar D. Rakić,
  8. and Peter Jacobson
Superconducting quantum circuits are one of the leading quantum computing platforms. To advance superconducting quantum computing to a point of practical importance, it is critical
to identify and address material imperfections that lead to decoherence. Here, we use terahertz Scanning Near-field Optical Microscopy (SNOM) to probe the local dielectric properties and carrier concentrations of wet-etched aluminum resonators on silicon, one of the most characteristic components of the superconducting quantum processors. Using a recently developed vector calibration technique, we extract the THz permittivity from spectroscopy in proximity to the microwave feedline. Fitting the extracted permittivity to the Drude model, we find that silicon in the etched channel has a carrier concentration greater than buffer oxide etched silicon and we explore post-processing methods to reduce the carrier concentrations. Our results show that near-field THz investigations can be applied to quantitatively evaluate and identify potential loss channels in quantum devices.
arxiv pdf