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 have developed a recently proposed concept of a Josephson traveling-wave parametric amplifier with threewave mixing. The amplifier consists of a microwave transmission line formed by an array of nonhysteretic one-junction SQUIDs. These SQUIDs are flux-biased in a way that the phase drops across the Josephson junctions are equal to 90°. Such a onedimensional metamaterial possesses a large quadratic nonlinearity and zero cubic (Kerr-like) nonlinearity. This property allows phase matching and exponential power gain to take place over a wide frequency range. The proof-of-principle experiment performed at a temperature of T = 4.2 K on Nb trilayer samples has demonstrated that our concept of a practical broadband Josephson parametric amplifier is valid and very promising for achieving quantum-limited performance.

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The paper reports on recent developments in a new technology process in LTS implementation to fabricate intrinsically shunted tunnel junctions. The process has been realized in SINIS multilayer thin-film technology. In various test series, circuits containing a large variety of single junctions and junction arrays of different contact areas and sizes were fabricated and measured. By variation of the oxidation parameters the fabrication process has been optimized for application in integrated circuits operating in RSFQ impulse logic. The junction parameter values realized for the critical current density range to up to about , those for the characteristic voltage to up to about . The junctions show nearly non-hysteretic current-voltage characteristics; the intra-wafer parameter spread is below 10%. The junctions realized fulfil the requirements imposed for digital RSFQ circuit operation at clock frequencies in the lower GHz frequency range.

We demonstrate that shunting of Superconductor-Insulator-Superconductor (S-I-S) Josephson junctions by Superconductor-Insulator-Normal metal (S-I-N) structures having pronounced nonlinear I-V characteristics can remarkably modify the Josephson dynamics. In the regime of Josephson generation the phase behaves as an overdamped coordinate, while in the superconducting state the damping and current noise are strikingly small, that is vitally important for application of such junctions for readout and control of Josephson qubits. Superconducting Nb/AlOx/Nb junction shunted by Nb/AlOx/AuPd junction of S-I-N type was fabricated and, in agreement with our model, exhibited non-hysteretic I-V characteristics at temperatures down to at least 1.4 K.

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