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Regular version of the site

Article

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

Superconductor Science and Technology. 2015. Vol. 28. P. 025009-25013.
Shcherbakova A., Fedorov K., Shulga K., Ryazanov V., Bolginov V., Oboznov V., Egorov S., Shkolnikov V., Wolf M., Beckmann D., Ustinov A.

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 π-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 π-shifters and measured their magnetic field response. The observed offset between the field responses of the qubits with and without π-junction is attributed to the π phase shift. The reported approach can be used for implementing a variety of hybrid Nb/Al superconducting quantum circuits.