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IEEE Transactions on Applied Superconductivity

Phase mode Josephson computer system

Kiyotaka Nakajima, H. Mizusawa, H. Sugahara, Yasuji Sawada

Published Mar 1, 1991• 75 citations
Publisher

Abstract

A discussion is presented of one of the simplest models of Josephson computer systems (phase mode system), in which quantized vortices of magnetic flux (fluxoids) are employed as information bits and in which all logic functions are achieved by interactions between fluxoids. In a phase mode system, device operation depends on the existence of many stable states differing from each other by integer multiples of 2 pi in the phase plane. As an example, the authors propose a simple model of a complete phase mode computing system and discuss the basic computer elements and the instruction in this system. The total system is considered to be a prototype of quantum computer systems in which physical quantum states are employed as logic states for information processing.>

DOI
10.1109/77.80746

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A discussion is presented of one of the simplest models of Josephson computer systems (phase mode system), in which quantized vortices of magnetic flux (fluxoids) are employed as information bits and in which all logic functions are achieved by interactions between fluxoids. In a phase mode system, device operation depends on the existence of many stable states differing from each other by integer multiples of 2 pi in the phase plane. As an example, the authors propose a simple model of a complete phase mode computing system and discuss the basic computer elements and the instruction in this system. The total system is considered to be a prototype of quantum computer systems in which physical quantum states are employed as logic states for information processing.>
We present experimental results on elementary phase-mode Josephson circuits whose combinations enable us to construct a total data processing system which is expected to be superior to the ordinary voltage-mode Josephson computer in several respects. In the phase-mode system a device depends for its operation on the existence of many stable states differing from one another by integer multiples of 2π in the phase plane. The total system is considered to be a prototype of quantum computer systems where physical quantum states are employed as logic states of information processing. By using the fabricated elementary circuits composed of SQUIDs and two types of branching points we have experimentally confirmed and, fan-out, fan-in operations, etc. We have also proposed an inhibit circuit, and presented the experimental results on the inhibit circuit.
We have theoratically and experimentally investigated the possibility of the phase mode operation of symmetrical four-junction Josephson interferometers (4J-loop), which are considered to play a key role as quantum logic devices for a Josephson switching system utilizing single-quanta as information bits, because of their excellent characters such as the kinetic momentum quantum effect and variety of the logic operations. The threshold characteristic, which is the most basic for Josephson interferometers to be operated in the phase mode and shows their fluxoid-quantum states, has experimentally been observed clearly for the first time for symmetrical 4J-loop's by using a novel radial-scanning, intensity-modulation technique, as predicted theoretically. It is characterized by a limited closed-shaped pattern. The number of the fluxoid-quantum states increases with LI0for the loops. Two different basic types of switching behaviors, a vortex-to-vortex and a vortex-to-voltage transition, have been confirmed, which have clearly been separated on the threshold characteristic, for the loops with LI0larger than\sim0.7\Phi_{0}The optimum LI0value for the phase mode operation of the loops has been found to be0.75\Phi_{0}\lsim LI_{0}\lsim 1.25\Phi_{0}. The desired performance of the phase mode operation of symmetrical 4J-loop has been confirmed by the shift motion of single fluxoids on the basis of the vortex-to-vortex transition of the loop. Finally, an application of such phase mode Josephson interferometers for pulse generators of Josephson sampling systems has been proposed.
The transition to Josephson digital circuits with the representation of information in the form of phase jumps of the superconducting order parameter on heterostructures promises a radical increase in the degree of integration while maintaining high speed and energy efficiency. However, it is not yet possible to manufacture reproducible bistable Josephson junctions, which are necessary for the functioning of the previously proposed basic units of the phase logic. To solve this problem, the concept of phase logic based on π junctions is proposed and analyzed within the resistive model of Josephson heterostructures. The potential energy of such junctions has a single minimum, with a difference in the order parameters of the electrodes equal to π. It is demonstrated that the use of π junctions allows one to implement the entire set of logic devices necessary for the operation of digital computing devices based on phase logic.
Quantum-state engineering, i.e., active control over the coherent dynamics of suitable quantum-mechanical systems, has become a fascinating prospect of modern physics. With concepts developed in atomic and molecular physics and in the context of NMR, the field has been stimulated further by the perspectives of quantum computation and communication. Low-capacitance Josephson tunneling junctions offer a promising way to realize quantum bits (qubits) for quantum information processing. The article reviews the properties of these devices and the practical and fundamental obstacles to their use. Two kinds of device have been proposed, based on either charge or phase (flux) degrees of freedom. Single- and two-qubit quantum manipulations can be controlled by gate voltages in one case and by magnetic fields in the other case. Both kinds of device can be fabricated with present technology. In flux qubit devices, an important milestone, the observation of superpositions of different flux states in the system eigenstates, has been achieved. The Josephson charge qubit has even demonstrated coherent superpositions of states readable in the time domain. There are two major problems that must be solved before these devices can be used for quantum information processing. One must have a long phase coherence time, which requires that external sources of dephasing be minimized. The review discusses relevant parameters and provides estimates of the decoherence time. Another problem is in the readout of the final state of the system. This issue is illustrated with a possible realization by a single-electron transistor capacitively coupled to the Josephson device, but general properties of measuring devices are also discussed. Finally, the review describes how the basic physical manipulations on an ideal device can be combined to perform useful operations.
We have investigated the possibility of the phase mode operation, i.e. the mode to transfer the fluxoid, not to transfer the voltage pulse, of Josephson switching system by fabricating a resistively coupled D.C. SQUID's array and 4J-loop which are expected to be advantageous in miniaturization by using its large kinetic momentum effect. The threshold characteristics of these superconducting loops and its LIcdependence agreed with the theoretical predictions. The phase mode logic actions were verified to operate really in a resistively coupled D.C.SQUID's and a coupled D.C. SQUID with a 4J-loop. A computer simulation showed that a resistively coupled SQUID's array acts both on phase mode and voltage mode by changing the circuit parameters, but that the two modes are clearly separated in a parameter space. Finally we have shown a total system configuration of the phase mode operation. For the power loss and the information processing speed, the phase mode was verified to have an advantage over the ordinary voltage mode by two orders of magnitude.