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IEEE Transactions on Magnetics

Fluxoid motion in phase mode Josephson switching system

Kiyotaka Nakajima, Gin-ichiro Oya, Yasuji Sawada

Published May 1, 1983• 22 citations
Publisher

Abstract

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.

DOI
10.1109/tmag.1983.1062418

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•Aug 21, 2015•0 citations
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.
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.
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.
It is known that SQUID has three important parameters, namely, Stewart-McCumber parameterβC, Screening parameterβLand thermal noise parameterΓ. They play a very important role in SQUID performance, especiallyβC. Recently, researchers extended the SQUID operating area toβc > 1 due to a large noise parameterΓin experiments. WhenβC> 1, SQUID exhibits a large flux-voltage transfer coefficient∂V/∂Φ, which is beneficial in reducing the noise contribution of the readout circuitδΦe. From the simulation results, we found that whenβC>1, SQUID produces a larger∂V/∂Φ thanβC∂V/∂Φ increases with the decrease ofβL. In order to further verify our findings, we selected two groups of SQUIDs for experiments (βCβC> 1), by changing the value of the shunt resistance on the Josephson junction to adjust the value of changingβC, and then we observe the value of∂V/∂Φ. Finally, we find that whenβL∂V/∂Φ of SQUID increases whileβCincreasing. A large∂V/∂Φ will help us to obtain an optimal SQUID measure system.
Digital Josephson circuits that do not latch into the voltage state provide an opportunity for high-speed serial processing. These fast logic gates must ultimately generate and record small fast pulses generated by single fluxoid events. This is demonstrated in a concept for a high-speed analog-to-digital (A/D) converter. Low β-value unlatched SQUID's are used throughout for ultimate speed, sensitivity, and simplicity. The analog signal is quantized by a single-junction SQUID and converted into a pulse train resulting from fluxoid transitions. These are counted in a chain of bistable two-junction SQUID's which successively scale the pulse train by a factor 2. These SQUID's perform both logic and memory functions at the single fluxoid level. The bit count stored in the array is gated out each sampling interval to provide the digitized signal. The performance of this device has been simulated and criteria established for the unlatched SQUID design. These include dimensionality, impedance, damping, intra- and inter-SQUID delays and β factor. Large analog dynamic range is available with the high intrinsic linearity of the SQUID quantizer and a long chain of scalers.
The dynamic properties of a superconducting quantum interference device (SQUID) containing arrays of Josephson junctions in a superconducting loop, and in particular the case of a four-junction SQUID, is analyzed theoretically via computer simulations. It is shown that phase locking of Josephson junctions determines the dynamic behavior of the SQUID. In the case of a stable phase-lock state hysteretic I-V curves as well as unusual voltage-flux dependencies appear. The influence of a small spread in the Josephson junction parameters upon the stability of the phase-lock state is investigated in parameter space.
The standard operation of a dc SQUID leads to oscillatory electric fields that emit electromagnetic radiation and can change the state of the measured sample. A stationary SQUID could be advantageous when back action on the measured sample has to be avoided. We study a superconducting loop that encloses a magnetic flux, connected to a superconducting and to a normal electrode, when a fixed electric current between the electrodes flows through the loop. The considered circuit does not contain Josephson junctions. We find that in a very broad range of parameters the current flow converges to a stationary regime. The potential difference between the electrodes depends on the magnetic flux, so that measuring this voltage would provide information on the enclosed flux. The influence of thermal noise was estimated. The sizes of the voltage and of the power dissipation could be appropriate for the design of a practical fluxmeter. We found narrow ranges of flux at which the voltage varies sharply with the flux.