Madisen Holbrook

Coherent and compact van der Waals transmon qubits

  1. Jesse Balgley,
  2. Jinho Park,
  3. Xuanjing Chu,
  4. Jiru Liu,
  5. Madisen Holbrook,
  6. Kenji Watanabe,
  7. Takashi Taniguchi,
  8. Archana Kamal,
  9. Leonardo Ranzani,
  10. Martin V. Gustafsson,
  11. James Hone,
  12. and Kin Chung Fong
State-of-the-art superconducting qubits rely on a limited set of thin-film materials. Expanding their materials palette can improve performance, extend operating regimes, and introduce
new functionalities, but conventional thin-film fabrication hinders systematic exploration of new material combinations. Van der Waals (vdW) materials offer a highly modular crystalline platform that facilitates such exploration while enabling gate-tunability, higher-temperature operation, and compact qubit geometries. Yet it remains unknown whether a fully vdW superconducting qubit can support quantum coherence and what mechanisms dominate loss at both low and elevated temperatures in such a device. Here we demonstrate quantum-coherent merged-element transmons made entirely from vdW Josephson junctions. These first-generation, fully crystalline qubits achieve microsecond lifetimes in an ultra-compact footprint without external shunt capacitors. Energy relaxation measurements, together with microwave characterization of vdW capacitors, point to dielectric loss as the dominant relaxation channel up to hundreds of millikelvin. These results establish vdW materials as a viable platform for compact superconducting quantum devices.
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