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IEEE Transactions on Magnetics

An elementary logic circuit employing superconducting Josephson tunneling gates

W.H. Henkels

Published Sep 1, 1974• 29 citations
Publisher

Abstract

The design and experimental investigation of a simple logic circuit employing Josephson tunnel junctions as the switching elements is described. The design utilized matched transmission lines as signal carriers. The physical layout was simulated on a computer with a model consisting of 3 Josephson gates and 28 transmission line segments. Results of both quasi-static and high speed testing were found to be in essential agreement with theoretical expectations; only dynamic results are presented here in detail. The experimental circuit had gates with 165 ps risetimes, and in the high speed investigation, was operated with bursts of 700 ps risetime pulses tightly spaced so as to simulate a repetition rate of ∼1 GHz.

DOI
10.1109/tmag.1974.1058406

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The design and experimental investigation of a simple logic circuit employing Josephson tunnel junctions as the switching elements is described. The design utilized matched transmission lines as signal carriers. The physical layout was simulated on a computer with a model consisting of 3 Josephson gates and 28 transmission line segments. Results of both quasi-static and high speed testing were found to be in essential agreement with theoretical expectations; only dynamic results are presented here in detail. The experimental circuit had gates with 165 ps risetimes, and in the high speed investigation, was operated with bursts of 700 ps risetime pulses tightly spaced so as to simulate a repetition rate of ∼1 GHz.
Flux quantum logic and memory circuits using superconducting Josephson tunnel junctions have high-speed switching times (approximately 1 ps), low power dissipation (< 1 (mu) W per circuit) and low levels of thermally induced electrical noise. Current designs of such circuits employ single trilayer junctions, which impose circuit size and logic threshold limitations. A new design component, the multilayered tunnel junction, consists of a vertically stacked array (a 1D superlattice) of Josephson tunnel junctions. The introduction of multilayered junctions into superconducting electronic circuitry offers a reduction in the current device size, fault tolerances, and new device applications. We present numerical simulations of simple circuits employing multilayered Josephson junctions as design components. Comparison with conventional single flux quantum circuitry is discussed. We also present preliminary measurements of multilayered Josephson junctions fabricated for use in flux quantum devices.
We have developed a method of designing single-flux-quantum (SFQ) logic circuits with passive gate-to-gate interconnections. Based on our method, we designed a 2/spl times/2 switch in which all the interconnections are implemented with passive transmission lines (PTLs) while short Josephson transmission line (JTL) segments are used only to adjust the signal timings. Compared with an identical switch using JTL interconnections, the switch using PTL interconnections has 45% fewer wiring junctions and requires 48% less wiring power current. The switch operated at 40 GHz with a bias margin of /spl plusmn/9.5%.
The potential of a new technology, based on the superconducting Josephson tunneling effect, has been assessed for use in high-speed computer systems. Josephson tunneling circuits for memory and logic functions can be switched at sub-nanosecond delay, do not require standby power, and dissipate extremely low energy, typically less than 10-13joule during fast switching operation. High-speed circuits with high packing densities, not limited by power dissipation problems, can therefore be expected. Low thermal noise at cryogenic temperatures ensures reliable operation despite the low energy switching signal. Practically lossless signal transmission via superconducting lines can be exploited for high-speed operation even when sizeable memory and logic modules are assembled. Two random access memory modules have been designed on the basis of single device experiments. Operational characteristics leading to the conclusion that 30 Mbit capacity, less than 1 watt refrigeration requirement (at 3.6° K) and cycle times of 40 ns (in one case) and 15 ns (in the second case), can be achieved are presented.
The behavior of simple superconducting circuits in the picosecond regime is described in a comprehensive way, with primary emphasis being given to the step function and pulse responses of these circuits. Topics receiving detailed discussion include Josephson-junction modeling with both the microscopic and shunted-junction models. Limitations of the shunted-junction model are explored by comparing it with experimental results and with the microscopic model. An approximate evaluation is given of the important dynamical properties of junctions made with the dominant fabrication technology (Pb-alloy systems), as a function of tunneling barrier thickness. Rounding out the device aspects of the discussion, we describe in detail the properties of superconducting microstrip transmission lines, with an emphasis on their high-speed behavior. Turning to simple circuits we review experimental results on the measurement of picosecond regime transient signals. The concept of turn-on delay is analyzed anew, providing simplified and extended results. Details of concepts for pulse height and pulsewidth measurements are explored, leading to the conclusion that the time resolution of superconducting circuits is limited to approximately the period of one plasma oscillation. With present Pb-alloy fabrication technology this limit is 2 ps.
A quantum-flux-type logic circuit is proposed which is composed of Josephson junction transmission lines, along which localized magnetic flux can propagate. By choosing bias current properly, the duplication of magnetic flux and a variety of logical functions can be obtained without changing the circuit topology. Computer simulation results are presented on AND and OR operations with two and three inputs for the same circuit topology, confirming that these circuits can be used as logic circuits. The simulations demonstrate the high-speed operation and low power consumption of this circuit.>
Using of traditional logic elements in digital electronics has ensured the development of a wide range of electronic devices for many years. In parallel, research and development of logical elements are carried out on the basis of other non-traditional physical phenomena or effects, in particular logic elements based on the phenomenon of superconductivity, the prospect of which is the use of very small energy consumption and ultrahigh performance. For superconducting logic elements, using various Josephson cryoelectronic structures, known as Josephson cryotrons, based on the stationary and non-stationary effects of Josephson. On the basis of the Josephson cryotrons, one can create Josephson elements of computer memory, and Josephson elements of digital logic. The main requirements for the Josephson cryotrons are a stable operating mode and high speed or short switching time. Information about the mode of operation and the speed give us the transition characteristics of the cryotrons - the time dependence of the voltage on the cryotron while changing its logical state, which can be obtained either experimentally, or theoretically. This work aims at a search for new ways of increasing the switching rate of digital logical operators by employing physical structures other than the traditional semiconductor-based schemes. We propose the principles of designing digital logical operators based on Josephson cryotrons, whose operation utilizes the stationary and dynamical Josephson effects, and describe the operational principles of the logical elements “AND” and “OR” bases on tunneling Josephson junctions “superconductor-insulator-superconductor”. Our proposed mathematical models for the commutation processes in such logical elements allowed us to calculate their transition characteristics during the switching and to determine the main parameters in such models. It was shown that the logical elements “AND” and “OR” can be implemented on individual cryotrons and that their logical state can be controlled by input signals in a form of current pulses. Such logical elements meet all the requirements for digital logical elements and have switching time of about 2-3 ps, which indicates their significantly increased switching rate.