Sein Park

Encapsulation epitaxy of air-stable monolayer superconducting films for quantum circuits and qubits

  1. Xudong Zheng,
  2. Sameia Zaman,
  3. Kenan Zhang,
  4. Connor A Occhialini,
  5. Haowei Xu,
  6. Zhien Wang,
  7. Fangyuan Liu,
  8. Luiz Gustavo Pimenta Martins,
  9. Sejoon Lim,
  10. Tianyi Zhang,
  11. Tilo H. Yang,
  12. Jiangtao Wang,
  13. Yunyue Zhu,
  14. Zachariah Hennighausen,
  15. Sein Park,
  16. Steven Vitale,
  17. Kevin Tibbetts,
  18. Stephen Margiotta,
  19. Phillip Kim,
  20. Cong Su,
  21. Ju Li,
  22. Riccardo Comin,
  23. William D. Oliver,
  24. Joel I.J. Wang,
  25. and Jing Kong
Two-dimensional (2D) superconductors are an emerging platform for strongly correlated physics and quantum information science. Their reduced dimensionality, atomically flat interfaces,
and high crystallinity are attractive for realizing compact lumped-element devices in superconducting circuits. However, synthesizing large-area, monolayer 2D superconductors remains challenging because of their susceptibility to oxidation. Here, we report an „encapsulation epitaxy“ mechanism that enables the growth of large-area, air-stable, monolayer superconducting NbSe2 films and explore their use in superconducting quantum circuits. A 2D encapsulation layer, such as graphene or hexagonal boron nitride (hBN), pre-deposited on a 3D substrate (e.g., SiO2 or Si3N4), serves both as a template for epitaxial growth of monolayer NbSe2 (1L-NbSe2) underneath it and as a protective cover. This approach produces uniform, large-area (>1-inch) 1L-NbSe2 with greatly enhanced ambient stability, enabling device fabrication in air. The resulting 1L-graphene/NbSe2 heterostructures exhibit robust superconductivity (Tc ~ 1 K) and enhanced charge density wave order (TCDW ~ 177 K), indicative of high material quality. We further integrate 1L-NbSe2 into superconducting circuits using oxidation-free transfer and superconducting edge-contact techniques. The 1L-NbSe2 exhibits a measured kinetic inductance LK ~ 0.7 nH/square, making it suitable for quantum circuits requiring high-kinetic-inductance elements. Encapsulation epitaxy thus provides a route to air-stable 2D superconductors and van der Waals heterostructures, with potential for wafer-scale, monolithic fabrication of superconducting quantum circuitry.
arxiv pdf

An All-van-der-Waals Qubit

  1. Sein Park,
  2. Sameia Zaman,
  3. Junghyun Kim,
  4. Junyoung An,
  5. Daniel Rodan-Legrain,
  6. Hung-Yu Tsao,
  7. Chia-Chin Tsai,
  8. Aranya Goswami,
  9. Réouven Assouly,
  10. William P. Banner,
  11. Gabriel D. Cutter,
  12. Kenji Watanabe,
  13. Takashi Taniguchi,
  14. Terry P. Orlando,
  15. Gil-Ho Lee,
  16. Kyle Serniak,
  17. Max Hays,
  18. Jeffrey A. Grover,
  19. Philip Kim,
  20. Pablo Jarillo-Herrero,
  21. Joel I.J. Wang,
  22. and William D. Oliver
Advances in solid-state physics, materials science, and device engineering have accelerated the development of superconducting qubits. Among emerging platforms, van der Waals (vdW)
materials and their heterostructures are potentially attractive building blocks for quantum devices, yet their realization in qubit architectures remains largely underexplored. Here we report an all-vdW superconducting qubit based on a NbSe2-hBN-NbSe2 junction, in which a thin hBN layer simultaneously provides Josephson coupling and capacitive shunting between two NbSe2 islands, forming a „merged-element“ transmon. Temporal characterization using circuit quantum electrodynamics (cQED) techniques yields an average energy-relaxation time T1,avg=55 μs, Hahn-echo coherence time T2E,avg=21 μs, and Ramsey coherence time T2R,avg=1.9 μs. The relatively low Ramsey time is primarily attributable to an enhanced sensitivity to charge noise consistent with the realized device parameters and not a fundamental limitation. These results show that lumped-element superconducting qubits based on vdW heterostructures can achieve coherence times comparable to those of conventional Al-AlOx-Al qubits, while offering a reduced device footprint and suppressed stray capacitive coupling.
arxiv pdf