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

摘要

We describe fabrication and testing of composite flux qubits combining Nb-and Al-based superconducting circuit technology. This hybrid approach to makingqubits allows for employing pi-phase shifters fabricated using well-establishedNb-based technology of superconductor-ferromagnet-superconductor Josephsonjunctions. The important feature here is to obtain high interface transparencybetween Nb and Al layers without degrading sub-micron shadow mask. We achievethis by in-situ Ar etching using e-beam gun. Shadow-evaporated Al/AlOx/AlJosephson junctions with Nb bias pads show the expected current-voltagecharacteristics with reproducible critical currents. Using this technique, wefabricated composite Nb/Al flux qubits with Nb/CuNi/Nb pi-shifters and measuredtheir magnetic field response. The observed offset between the field responsesof 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/Alsuperconducting quantum circuits.