The d-wave pairing symmetry in high-critical temperature superconductors makes it possible to realize superconducting rings with built-in pi phase shifts. Such rings have a twofold degenerate ground state that is characterized by the spontaneous generation of fractional magnetic flux quanta with either up or down polarity. We have incorporated pi phase-biased superconducting rings in a logic circuit, a flip-flop, in which the fractional flux polarity is controllably toggled by applying single flux quantum pulses at the input channel. The integration of p rings into conventional rapid single flux quantum logic as natural two-state devices should alleviate the need for bias current lines, improve device symmetry, and enhance the operation margins.

The latching of temporary data is essential in the Rapid Single Flux Quantum (RSFQ) electronics family. Its pulse-driven nature requires two or more stable states in almost all cells. Storage loops must be designed to have exactly two stable states for binary data representation. In conventional RSFQ such loops are constructed to have two stable states, e.g. by using asymmetric bias currents. This bistability naturally occurs when phase-shifting elements are included in the circuitry, such as-Josephson junctions or a-phase shift associated with an unconventional (-wave) order parameter symmetry. Both approaches can be treated completely analogously, giving the same results. We have demonstrated for the first time the correct operation of a logic circuit, a toggle-flip-flop, using rings with an intrinsic-phase shift (-rings) based on hybrid high-to low-Josephson junctions. Because of their natural bistability these-rings improve the device symmetry, enhance operation margins and alleviate the need for bias current lines.

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We investigated the gray zone width of Josephson comparators by means of circuit simulations and experiments, looking at the dependences on different circuit parameters and topologies. Eight different comparator circuits were simulated and designed for a 1 kA cm −2 niobium device. With our sophisticated measurement set-up, the lowest reported gray zone width of 3.2 μA at 4.2 K was measurable. Moreover, the results obtained allow us to derive a set of design rules for further reduction of the gray zone width, which was the original goal of our investigations.