Xuanjing Chu

Measuring Reactive-Load Impedance with Transmission-Line Resonators Beyond the Perturbative Limit

  1. Xuanjing Chu,
  2. Jinho Park,
  3. Jesse Balgley,
  4. Sean Clemons,
  5. Ted S. Chung,
  6. Kenji Watanabe,
  7. Takashi Taniguchi,
  8. Leonardo Ranzani,
  9. Martin V. Gustafsson,
  10. Kin Chung Fong,
  11. and James Hone
We develop an analytic framework to extract circuit parameters and loss tangent from superconducting transmission-line resonators terminated by reactive loads, extending analysis beyond
the perturbative regime. The formulation yields closed-form relations between resonant frequency, participation ratio, and internal quality factor, removing the need for full-wave simulations. We validate the framework through circuit simulations, finite-element modeling, and experimental measurements of van der Waals parallel-plate capacitors, using it to extract the dielectric constant and loss tangent of hexagonal boron nitride. Statistical analysis across multiple reference resonators, together with multimode self-calibration, demonstrates consistent and reproducible extraction of both capacitance and loss tangent in close agreement with literature values. In addition to parameter extraction, the analytic relations provide practical design guidelines for maximizing energy participation ratio in the load and improving the precision of resonator-based material metrology.
arxiv pdf

Crystalline superconductor-semiconductor Josephson junctions for compact superconducting qubits

  1. Jesse Balgley,
  2. Jinho Park,
  3. Xuanjing Chu,
  4. Ethan G. Arnault,
  5. Martin V. Gustafsson,
  6. Leonardo Ranzani,
  7. Madisen Holbrook,
  8. Kenji Watanabe,
  9. Takashi Taniguchi,
  10. Vasili Perebeinos,
  11. James Hone,
  12. and Kin Chung Fong
The narrow bandgap of semiconductors allows for thick, uniform Josephson junction barriers, potentially enabling reproducible, stable, and compact superconducting qubits. We study vertically
stacked van der Waals Josephson junctions with semiconducting weak links, whose crystalline structures and clean interfaces offer a promising platform for quantum devices. We observe robust Josephson coupling across 2–12 nm (3–18 atomic layers) of semiconducting WSe2 and, notably, a crossover from proximity- to tunneling-type behavior with increasing weak link thickness. Building on these results, we fabricate a prototype all-crystalline merged-element transmon qubit with transmon frequency and anharmonicity closely matching design parameters. We demonstrate dispersive coupling between this transmon and a microwave resonator, highlighting the potential of crystalline superconductor-semiconductor structures for compact, tailored superconducting quantum devices.
arxiv pdf