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.
Accurate inductance calculations are critical for the design of both digital and analogue
superconductive integrated circuits, and three-dimensional calculations are gaining
importance with the advent of inductive biasing, inductive coupling and sky plane shielding
for RSFQ cells. InductEx, an extraction programme based on the three-dimensional
calculation software FastHenry, was proposed earlier. InductEx uses segmentation
techniques designed to accurately model the geometries of superconductive integrated
circuit structures. Inductance extraction for complex multi-terminal three-dimensional
structures from current distributions calculated by FastHenry is discussed. Results for
both a reflection plane modelling an infinite ground plane and a finite segmented
ground plane that allows inductive elements to extend over holes in the ground
plane are shown. Several SQUIDs were designed for and fabricated with IPHT’s
1 kA cm − 2
RSFQ1D niobium process. These SQUIDs implement a number of loop structures that
span different layers, include vias, inductively coupled control lines and ground plane
holes. We measured the loop inductance of these SQUIDs and show how the
results are used to calibrate the layer parameters in InductEx and verify the
extraction accuracy. We also show that, with proper modelling, FastHenry can
be fast enough to be used for the extraction of typical RSFQ cell inductances.
Integration with conventional electronics offers a straightforward and economical approach to upgrading existing superconducting technologies, such as scaling up superconducting detectors into large arrays and combining single flux quantum (SFQ) digital circuits with semiconductor logic and memories. However, direct output signals from superconducting devices (e.g., Josephson junctions) are usually not compatible with the input requirements of conventional devices (e.g., transistors). Here, we demonstrate the use of a single three-terminal superconducting-nanowire device, called the nanocryotron (nTron), as a digital comparator to combine SFQ circuits with mature semiconductor circuits such as complementary metal oxide semiconductor (CMOS) circuits. Since SFQ circuits can digitize output signals from general superconducting devices and CMOS circuits can interface existing CMOS-compatible electronics, our results demonstrate the feasibility of a general architecture that uses an nTron as an interface to realize a "super-hybrid" system consisting of superconducting detectors, superconducting quantum electronics, CMOS logic and memories, and other conventional electronics.
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