Fabrication and measurements of hybrid Nb/Al Josephson junctions and flux qubits with pi-shifters

  1. A. V. Shcherbakova,
  2. K. G. Fedorov,
  3. K. V. Shulga,
  4. V. V. Ryazanov,
  5. V. V. Bolginov,
  6. V. A Oboznov,
  7. S. V. Egorov,
  8. V. O. Shkolnikov,
  9. M. J. Wolf,
  10. D. Beckmann,
  11. and A. V. Ustinov
We describe fabrication and testing of composite flux qubits combining Nb- and Al-based superconducting circuit technology. This hybrid approach to making qubits allows for employing pi-phase shifters fabricated using well-established Nb-based technology of superconductor-ferromagnet-superconductor Josephson junctions. The important feature here is to obtain high interface transparency between Nb and Al layers without degrading sub-micron shadow mask. We achieve this by in-situ Ar etching using e-beam gun. Shadow-evaporated Al/AlOx/Al Josephson junctions with Nb bias pads show the expected current-voltage characteristics with reproducible critical currents. Using this technique, we fabricated composite Nb/Al flux qubits with Nb/CuNi/Nb pi-shifters and measured their magnetic field response. The observed offset between the field responses of the qubits with and without pi-junction is attributed to the pi phase shift. The reported approach can be used for implementing a variety of hybrid Nb/Al superconducting quantum circuits.
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