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IEEE Transactions on Magnetics

Evaluation of flux-based logic schemes for high-T/sub c/ applications

J. Fleischman, David A. Feld, P. Xiao, T. Van Duzer

Published Mar 1, 1991• 4 citations
Publisher

Abstract

Three digital logic families that can be made using nonhysteretic Josephson junctions (potentially the only kind of Josephson device realizable with superconductors having high transition temperatures) are analyzed. These logic families utilize magnetic flux-transfer, and are characterized by very low power dissipation. Rapid single flux quantum (RSFQ) and phase-mode logic are both based on pulse propagation. The quantum flux parametron (QFP) logic family is based on current latching. Simulations of RSFQ, Phase-Mode, and QFP logic families using high-T/sub c/ junction parameters are presented to demonstrate the compatibility of these logic families with the perovskite superconductors. The operation of these logic families is analyzed, and the advantages and disadvantages of each are discussed.

DOI
10.1109/20.133785

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Ultrafast Electronics and Optoelectronics (UEO)•Jan 1, 1995•0 citations
Three digital logic families that can be made using nonhysteretic Josephson junctions (potentially the only kind of Josephson device realizable with superconductors having high transition temperatures) are analyzed. These logic families utilize magnetic flux-transfer, and are characterized by very low power dissipation. Rapid single flux quantum (RSFQ) and phase-mode logic are both based on pulse propagation. The quantum flux parametron (QFP) logic family is based on current latching. Simulations of RSFQ, Phase-Mode, and QFP logic families using high-T/sub c/ junction parameters are presented to demonstrate the compatibility of these logic families with the perovskite superconductors. The operation of these logic families is analyzed, and the advantages and disadvantages of each are discussed.
Recent developments concerning the rapid single-flux-quantum (RSFQ) circuit family are reviewed. Elementary cells in this circuit family can generate, pass, memorize, and reproduce picosecond voltage pulses with a nominally quantized area corresponding to transfer of a single magnetic flux quantum across a Josephson junction. Functionally, each cell can be viewed as a combination of a logic gate and an output latch (register) controlled by clock pulses, which are physically similar to the signal pulses. Hand-shaking style of local exchange by the clock pulses enables one to increase complexity of the LSI RSFQ systems without loss of operating speed. The simplest components of the RSFQ circuitry have been experimentally tested at clock frequencies exceeding 100 GHz, and an increase of the speed beyond 300 GHz is expected as a result of using an up-to-date fabrication technology. This review includes a discussion of possible future developments and applications of this novel, ultrafast digital technology.>
A new family of dc-powered Josephson junction digital devices, the Rapid Single Flux Quantum (RSFQ) logic, is described. The devices use overdamped Josephson junctions and two-junction interferometers to store, pass and process the digital information presented in form of single flux quanta. We have carried out extensive numerical simulation of the dynamics of the RSFQ logic gates and of some more complex circuits including serial full adder and reversible shift register, within the standard microscopic-theory ("Werthamer") description of Josephson junctions. The minimum clock cycles of the basic RSFQ circuits turn out to be as small as 2.5 ps. The most promising ways to use the RSFQ logic circuits at the present stage of development of the Josephson junction digital technology are discussed.
The authors argue that superconducting digital electronics is the future trend for high-speed and low-power applications. Low-T/sub c/ technology based on stable Nb-AlO/sub x/-Nb tunneling junctions has been successful in constructing most of the digital circuit logic elements. A four-stage shift register constructed with latching logic operated up to 4 GHz with a power consumption of about 60 mu W per gate. A 12-b analog-digital coverter constructed with a single flux quantum (SFQ) logic showed proper operation with the total power dissipated being only about 100 mu W. Fabrication of Josephson tunneling junctions with high-T/sub c/ materials has been tougher than with low-T/sub c/ materials. However, recently developed very high speed rapid single flux quantum (RSFQ) logic circuits can be applied to weak link type junctions fabricated with YBa/sub 2/Cu/sub 3/O/sub 7-x/ films. Simulations of the various circuits with weak link type junctions, e.g., shift registers, analog-digital converters, etc., show fast operation and reasonable margins. Applications of similar concepts can be applied to NbN circuits operating at above 10 K.>
Abstract Superconductor logic families can operate with small power dissipation and are thus suitable as building blocks for various computing systems. In some applications, superconductor logic circuits should be designed using Josephson junctions with low I c values ( I c : critical current). For instance, lowering I c values enables qubit interface circuits to operate with very small power dissipation at ~10 mK and stochastic electronics to easily induce stochastic operations. In this study, we develop the AIST 1 kA cm −2 Nb planarized process (1KP) with a minimum critical current of 10 µA, dedicated to the design of qubit interface circuits and stochastic electronics. We also develop rapid single-flux-quantum (RSFQ) and adiabatic quantum-flux-parametron (AQFP) cell libraries using the 1KP. The power dissipation of RSFQ logic using the 1KP can be reduced to 3.2% of that for conventional RSFQ logic by reducing both I c values and a bias voltage. Furthermore, the amount of supply currents for AQFP circuits using the 1KP can be reduced to ~40% of that for conventional AQFP circuits due to a large mutual inductance between AQFP gates and excitation lines, which results from a reduction in I c and an increase in inductances. We demonstrate RSFQ and AQFP circuits fabricated by the 1KP at 4.2 K. These results indicate that RSFQ and AQFP circuits using the 1KP have the potential to be used for the design of qubit interface circuits and stochastic electronics.
Each superconductor logic family has distinct features and advantages. For instance, adiabatic quantum-flux-parametron (AQFP) circuits operate with very high energy efficiency, and rapid single-flux-quantum (RSFQ) circuits operate at very high clock frequencies. To build a hybrid system that combines the above advantages, we propose a compact interface between AQFP and RSFQ circuits that converts current signals in AQFP logic into flux signals in RSFQ logic using a non-adiabatic quantum-flux-parametron gate. Compared to a previously proposed AQFP/RSFQ interface, the proposed interface has far fewer Josephson junctions (4 vs. 11). We optimize the circuit parameters of the AQFP/RSFQ interface and demonstrate the interface at low speed as proof of concept. Furthermore, we demonstrate data propagation between AQFP circuits along a long passive transmission line via the AQFP/RSFQ interface at low speed to show that the AQFP/RSFQ interface can alleviate the interconnect length limit in AQFP circuits.