The difference between the phases of superconducting order parameter plays in superconducting circuits the role similar to that played by the electrostatic potential difference required to drive a current in conventional circuits. This fundamental property can be altered by inserting in a superconducting circuit a particular type of weak link, the so-called Josephson π-junction having inverted current-phase relation and enabling a shift of the phase by π. We demonstrate the operation of three superconducting circuits -two of them are classical and one quantum -which all utilize such π-phase shifters realized using superconductor-ferromagnet-superconductor sandwich technology. The classical circuits are based on single-flux-quantum cells, which are shown to be scalable and compatible with conventional niobium-based superconducting electronics. The quantum circuit is a π-phase biased qubit, for which we observe coherent Rabi oscillations and compare the measured coherence time with that of conventional superconducting phase qubits. 1 arXiv:1005.1581v1 [cond-mat.supr-con]

We report on the functionality of a Nb-based superconducting single flux quantum (SFQ) toggle flip-flop (TFF) circuit, comprising a complementary superconductor-ferromagnetsuperconductor (SFS) Josephson π-junction. The SFS junction was used as a phase shifting element inserted in the storage loop of the TFF. The fabricated circuits demonstrated correct functionality with the operation parameter ranges of ±20%. The application of SFS π-junctions makes the SFQ circuits very compact, may substantially improve their stability, and may also be suitable for integration with Josephson quantum circuits (qubits).

We report the observation of a current-phase relation dominated by the second Josephson harmonic in superconductor-ferromagnet-superconductor junctions. The exotic current-phase relation is realized in the vicinity of a temperature-controlled 0-to-π junction transition, at which the first Josephson harmonic vanishes. Direct current-phase relation measurements, as well as Josephson interferometry, non-vanishing supercurrent and half-integer Shapiro steps at the 0-π transition selfconsistently point to an intrinsic second harmonic term, making it possible to rule out common alternative origins of half-periodic behavior. While surprising for diffusive multimode junctions, the large second harmonic is in agreement with theory predictions for thin ferromagnetic interlayers.

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