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IEEE Transactions on Applied Superconductivity

Quantum flux parametron with magnetic flux regulator

Y. Harada, W. Hioe, Eiichi Goto

Published Jun 1, 1991• 3 citations
Publisher

Abstract

A magnetic flux regulator for a quantum flux parametron (QFP) is proposed. The regulator is formed with a parallel connection of a Josephson junction across activation inductors building an RF-SQUID (superconducting quantum interference device). This junction makes a nonlinear bypass channel for magnetic activation flux and contributes to regulation of applied flux and elimination of undesirable external noises. Using this flux regulator, the activation condition tolerance is not only improved, but also a higher speed transition from a nonactivated state to an activated state is expected. QFP operations with the regulators have been observed with integrated circuits.>

DOI
10.1109/77.84614

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IEEE Transactions on Magnetics•Mar 1, 1991•8 citations
A magnetic flux regulator for a quantum flux parametron (QFP) is proposed. The regulator is formed with a parallel connection of a Josephson junction across activation inductors building an RF-SQUID (superconducting quantum interference device). This junction makes a nonlinear bypass channel for magnetic activation flux and contributes to regulation of applied flux and elimination of undesirable external noises. Using this flux regulator, the activation condition tolerance is not only improved, but also a higher speed transition from a nonactivated state to an activated state is expected. QFP operations with the regulators have been observed with integrated circuits.>
The Quantum Flux Parametron (QFP) is a Josephson Junction device using the polarity of a quantum magnetic flux to represent a binary information. A quantum unit of magnetic flux in a QFP can be controlled to realize majority logic operation, consequently AND and OR logics can be realized. Together with negation, which can be realized by a flux-inverting transformer, various computing logic devices can be constructed. In addition, the QFP is also a latching device so that a flux can be confined in the device to realize memory function. Due to the homogenous nature of memory and logic circuits as well as the flux nature of the signal which can be transmitted inductively, an entirely new Josephson supercomputer with memory and CPU pipelining arranged in a 3D setting was conceived. The computer with 40 processors is expected to run at a maximum speed of 20 GFLOPS of scalar processing.
The quantum-flux parametron (QFP) is a single-flux quantum Josephson device with a high gain of over 20 dB and a low power dissipation of 1-nW order at 10 GHz. Logic and analog circuits using the device have been fabricated and tested. The device's margin may be reduced by a number of problems due to fabrication and design variations such as imbalance of Josephson-junction critical currents, flux loss in transformer self-inductances, and transformer coupling variations. The authors review these problems and describe three auxiliary circuits-booster, puller, and regulator-for alleviating them. A booster reduces the effect of critical current variations and doubles the output. A puller increases the activation current, thus allowing smaller clock signals. A regulator ensures full activation of the QFP over a wide range of activation levels. With these currents, the device has a much improved margin against variations originating from within itself.>
Quantum Flux Parametron (QFP) logic gates were designed and fabricated from a single layer of YBa/sub 2/Cu/sub 3/O/sub 7/ thin film based on step-edge Josephson junctions. Self-inductance (L) of the QFP loop, an important circuit parameter, as is critical current (J/sub c/) of the Josephson junction, was determined by using directly coupled dc superconducting quantum interference devices (SQUIDs). Magnetically coupled dc SQUIDs were used to detect the output signal from the QFP. Simulated waveforms were used to identify the operation modes of the QFP as a function of the circuit parameters. From examination of the output waveforms of the QFP, operation in agreement with simulations was confirmed.
A flash-type superconducting A/D (analog/digital) converter is proposed. The basis low-frequency operation of this converter, which compromises a quantum flux parametron (QFP) and an RF SQUID (superconducting quantum interference device), has been observed. The RF SQUID consists of a Josephson junction and a load inductor which is part of the QFP. It converts an analog signal to period digital signals of one quantum flux (2.07*10/sup -5/ W-b) per cycle. The QFP works as both an amplifier and a comparator of the periodic digital signals, by adding an exciting signal. This A/D converter is expected to achieve high-speed operation of 20 GHz. Several 4-bit A/D converter circuits were fabricated, using the conventional Nb-Pb alloy process, with a junction size of 5 mu m*5 mu m and supercurrent density of 200 A/cm/sup 2/. Basic operation at 100 KHz was confirmed experimentally.>
Flux transfer device theory is reviewed. In such devices, generalized magnetic flux, defined as the time integral of voltage, is used to describe the device characteristics. The Josephson junction and inductor are the main circuit elements in flux transfer devices, because they maintain a constant value for the time integral of voltage. Flux transfer devices are based on either an RF SQUID (superconducting quantum interference device) or a fluxon device. Four devices, including the parametric quantron and the quantum flux parametron, are reviewed as applications of the RF SQUID. The fluxon feedback oscillator and a soliton device are also reviewed as applications of fluxon devices. The quantum flux parametron is then described. The parametron principle and the fundamental properties of the quantum flux parametron such as gain, switching speed, and power dissipation are discussed. Logic circuits and a memory cell are also reviewed. A novel analog-to-digital converter is proposed as an application of the quantum flux parametron.>
The quantum flux parametron (QFP) is a flux-transfer, flux-activated Josephson logic device which realizes much lower power dissipation than other Josephson logic devices. Being a two-terminal device, its correct operation may be affected by coupling to other QFPs. The problems include backcoupling form active QFPs through inactive QFPs (relay noise), coupling between QFPs activated at different times because of clock skew (homophase noise), and interaction between active QFPs (reaction hazard). Previous QFP circuits worked by wired-majority, which is a linear input logic and has low input margin. A logic gate (D-gate) using a QFP to perform logic operations has been analyzed and tested by computer simulation. Relay noise, homophase noise, and reaction hazard are substantially reduced. Moreover, the inputs have little interaction, and hence input margin is greatly improved. Using only D-gates it is possible to realize any combinational logic function. Important logic functions can be realized using one gate, such as a two-input multiplexer, majority, and parity. Since the QFP has a latching effect, D-gates can realize fully pipelined circuits without additional registers.