Harsh Bhardwaj

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

Thermal reconstruction as a method of substrate preparation for highly crystalline superconducting TiN resonators

  1. Thomas J. Smart,
  2. Marc Neis,
  3. Janine Lorenz,
  4. Marcello P. Guardascione,
  5. Roudy Hanna,
  6. Michael Schleenvoigt,
  7. Yuan Gao,
  8. Joscha Domnick,
  9. Benjamin Bennemann,
  10. Abdur Rehman Jalil,
  11. Jin Hee Bae,
  12. Harsh Bhardwaj,
  13. F. Stefan Tautz,
  14. Felix Lüpke,
  15. Detlev Grützmacher,
  16. Rami Barends,
  17. Pavel A. Bushev,
  18. and Peter Schüffelgen
High quality crystalline growth of a thin film on sapphire requires sufficient substrate preparation, often achieved via the use of aggressive chemical cleaning. Direct thermal reconstruction
of the sapphire substrate via a CO2 laser beam may allow for an alternative way to prepare the substrate for epitaxy without the use of any chemical processing. Within this work, we demonstrate that thermal annealing of sapphire into its (31‾‾‾√×31‾‾‾√)R±9° reconstruction is a valid alternative preparation technique for sapphire substrates. TiN films grown via plasma-assisted molecular beam epitaxy upon these substrates exhibit greater crystallinity than those grown on chemically cleaned sapphire substrates. Superconducting resonators fabricated from these films exhibit similar performance, with many possessing internal quality factors at single photon levels greater than 106 for both substrate preparation methods.
arxiv pdf