Integration von π-Kontakten in Josephson-Qubits und Einzelflussquanten-Strukturen

The remarkable property of exhibiting an additional phase drop of π found in the superconductor-ferromagnet-superconductor (SFS) Josephson junctions of certain thickness of the ferromagnetic layer [1] makes them promising for the implementation of Josephson qubits and Single Flux Quantum (SFQ) logic structures. The application of the SFS π-junctions may substantially improve stability of the phase qubits and make SFQ circuitry particularly compact by replacing the large-size inductances. Moreover simulations of single flux quanta structures with integrated π shift elements demonstrate an enlarged operation margin. An additional advantage for these circuits is a strongly decreased power consumption.

Here we report the first data obtained on the integrated SFS+SFQ circuits manufactured in two laboratories (PTB, Braunschweig and ISSP, Chernogolovka) by co-fabricating the SFS junctions on the prepared Nb circuits. These Nb circuits were designed for and fabricated in the standard 4 µm Superconductor-Insulator-Superconductor (SIS) Nb/Al-AlO_x/Nb PTB trilayer technology process with a critical current density of jc = 100 A/cm² . The SFS junctions of type Nb/CuNi/Nb with lateral dimensions 5µm by 5µm and jc = 1 kA/cm² were co-fabricated on top of the Nb wiring layer. We have fabricated and experimentally demonstrated the correct operation of a dc interferometer with π-shift in its characteristics. We also developed a compact SFQ Toggle Flip-Flop (TFF) device, whose storing inductance had been replaced by a SFS π-junction. This work was supported by DFG.

[1] V.V. Ryazanov et al., Phys. Rev. Lett. 86, 2427 (2001).

Figure 1:
Left: Photograph of an interferometer circuit with integrated π-contact. 
Right: Flux-Voltage characteristic of the SQUID with integrated π-contact