James Hone

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

Miniaturizing transmon qubits using van der Waals materials

  1. Abhinandan Antony,
  2. Martin V. Gustafsson,
  3. Guilhem J. Ribeill,
  4. Matthew Ware,
  5. Anjaly Rajendran,
  6. Luke C.G. Govia,
  7. Thomas A. Ohki,
  8. Takashi Taniguchi,
  9. Kenji Watanabe,
  10. James Hone,
  11. and Kin Chung Fong
Quantum computers can potentially achieve an exponential speedup versus classical computers on certain computational tasks, as was recently demonstrated in systems of superconductingqubits. However, these qubits have large footprints due to the need of ultra low-loss capacitors. The large electric field volume of \textit{quantum compatible} capacitors stems from their distributed nature. This hinders scaling by increasing parasitic coupling in circuit designs, degrading individual qubit addressability, and limiting the minimum achievable circuit area. Here, we report the use of van der Waals (vdW) materials to reduce the qubit area by a factor of >1000. These qubit structures combine parallel-plate capacitors comprising crystalline layers of superconducting niobium diselenide (NbSe2) and insulating hexagonal-boron nitride (hBN) with conventional aluminum-based Josephson junctions. We measure a vdW transmon T1 relaxation time of 1.06 μs, demonstrating that a highly-compact capacitor can reach a loss-tangent of <2.83×10−5. Our results demonstrate a promising path towards breaking the paradigm of requiring large geometric capacitors for long quantum coherence in superconducting qubits, and illustrate the broad utility of layered heterostructures in low-loss, high-coherence quantum devices.[/expand]
arxiv pdf