Albert Hertel

Parametric two-qubit gates via Landau-Zener interference

  1. Simon Geisert,
  2. Albert Hertel,
  3. Sören Ihssen,
  4. Zhongyi Jiang,
  5. Paul Kugler,
  6. Nicolas Zapata,
  7. Nicolas Gosling,
  8. Ameya Nambisan,
  9. Yuan Gao,
  10. Asier Galicia,
  11. Jéferson R. Guimarães,
  12. Yorgo Haddad,
  13. Marc Neis,
  14. Harsh Bhardwaj,
  15. Dmitriy A. Volkov,
  16. Juan Cereijo,
  17. Marcello Guardascione,
  18. Yebin Liu,
  19. Markus Jerger,
  20. Pavel Bushev,
  21. Frank Wilhelm-Mauch,
  22. Wolfgang Wernsdorfer,
  23. Shai Machnes,
  24. Mohammad Ansari,
  25. Rami Barends,
  26. and Ioan M. Pop
We propose and demonstrate gates between two superconducting qubits based on quantum interference of consecutive Landau-Zener (LZ) transitions. This gate mechanism bridges between baseband
and parametric two-qubit control, enabling in situ tuning of the control frequency across a continuous interval up to hundreds of MHz. Another advantage compared to dispersive couplers is that the speed of the LZ gate is on the order of the full coupling strength. We experimentally demonstrate the gate on two platforms, a modular chiplet architecture of coupled generalized flux qubits, and on a monolithic transmon architecture. The combination of tunability and gate speed establishes the LZ gate as a unique tool for multiplexing control pulses and interconnecting superconducting chiplet architectures.
arxiv pdf

On-chip stencil lithography for superconducting qubits

  1. Roudy Hanna,
  2. Sören Ihssen,
  3. Simon Geisert,
  4. Umut Kocak,
  5. Matteo Arfini,
  6. Albert Hertel,
  7. Thomas J. Smart,
  8. Michael Schleenvoigt,
  9. Tobias Schmitt,
  10. Joscha Domnick,
  11. Kaycee Underwood,
  12. Abdur Rehman Jalil,
  13. Jin Hee Bae,
  14. Benjamin Bennemann,
  15. Mathieu Féchant,
  16. Mitchell Field,
  17. Martin Spiecker,
  18. Nicolas Zapata,
  19. Christian Dickel,
  20. Erwin Berenschot,
  21. Niels Tas,
  22. Gary A. Steele,
  23. Detlev Grützmacher,
  24. Ioan M. Pop,
  25. and Peter Schüffelgen
Improvements in circuit design and more recently in materials and surface cleaning have contributed to a rapid development of coherent superconducting qubits. However, organic resists
commonly used for shadow evaporation of Josephson junctions (JJs) pose limitations due to residual contamination, poor thermal stability and compatibility under typical surface-cleaning conditions. To provide an alternative, we developed an inorganic SiO2/Si3N4 on-chip stencil lithography mask for JJ fabrication. The stencil mask is resilient to aggressive cleaning agents and it withstands high temperatures up to 1200\textdegree{}C, thereby opening new avenues for JJ material exploration and interface optimization. To validate the concept, we performed shadow evaporation of Al-based transmon qubits followed by stencil mask lift-off using vapor hydrofluoric acid, which selectively etches SiO2. We demonstrate average $T_1 \approx 75 \pm 11~\SI{}{\micro\second}$ over a 200 MHz frequency range in multiple cool-downs for one device, and $T_1 \approx 44\pm 8~\SI{}{\micro\second}$ for a second device. These results confirm the compatibility of stencil lithography with state-of-the-art superconducting quantum devices and motivate further investigations into materials engineering, film deposition and surface cleaning techniques.
arxiv pdf