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Journal of Applied Physics

Dynamical properties of fluxon propagation in a Josephson transmission line with a trigger turning point

Akira Fujimaki, Koji Nakajima, Yasuji Sawada

Published Jun 15, 1987• 12 citations
Publisher

Abstract

Dynamical properties of fluxon propagation in a discrete Josephson transmission line are reported. The circuit consists of three Josephson transmission lines and a trigger turning point. By using the Josephson sampler, we confirmed fan-out operation of the phase-mode logic circuit in which single fluxons were employed as information bits. We measured the fluxon velocity propagating in the Josephson transmission line. The measured velocity of 3.38×107 m/s was larger than that of the continuous Josephson transmission line, and it is a manifestation of a remarkable feature of the discrete Josephson transmission line. The experimental results affirm the high-speed operation of the phase-mode circuits constructed by discrete Josephson transmission lines.

DOI
10.1063/1.338238

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Dynamical properties of fluxon propagation in a discrete Josephson transmission line are reported. The circuit consists of three Josephson transmission lines and a trigger turning point. By using the Josephson sampler, we confirmed fan-out operation of the phase-mode logic circuit in which single fluxons were employed as information bits. We measured the fluxon velocity propagating in the Josephson transmission line. The measured velocity of 3.38×107 m/s was larger than that of the continuous Josephson transmission line, and it is a manifestation of a remarkable feature of the discrete Josephson transmission line. The experimental results affirm the high-speed operation of the phase-mode circuits constructed by discrete Josephson transmission lines.
Dynamics of fluxons propagating in a Josephson transmission line (JTL) have been studied in real time using Josephson sampling techniques. A JTL is an NbN-oxide-NbN Josephson junction with dimensions of 2.5 × 150 μm2, while Josephson samplers and pulse generators for generating and driving fluxons are fabricated by a Pb-alloy technology. Among the various behaviors, especially, it is found that a propagating fluxon can be made to pause without reflections at any desired position which is created by placing a resistor on the surface of a portion of a JTL. The surface resistor has a length larger than the Josephson penetration depth. A pausing fluxon can be re-driven by supplying a small current to the portion of the surface resistor after more than 100 μs, The experimental results are compared with computer simulations. Reflections of fluxons observed by samplers have also been discussed.
We have directly observed the reflection of fluxons in a Josephson transmission line (JTL) with an open end using a Josephson sampler. Depending on the bias current at the end of the JTL, a reflected single fluxon, an antifluxon, and multi-antifluxons are observed with the measurements of the propagating time delay for the reflection process. The observed reflection properties, including the interaction between reflected antifluxons, are in fairly good agreement with numerical simulations taking account of the nonlinear shunting loss and the surface loss.
By numerical calculations of the differential-difference sine-Gordon equation, we have discussed the discrete Josephson-junction transmission lines which are constructed of a series of small-area Josephson junctions connected by superconducting strips. It is shown that the discrete Josephson lines containing D lines, N lines, T turning points, and S turning points are elementarily characterized by the discreteness parameter (2πLIc/Φ0)1/2. On the discrete Josephson logic circuits there exists a region of forbidden propagation in the (2πLIc/Φ0)1/2-γ (bias-current parameter) plane for single flux quanta. A single flux quantum can be stuffed in a small area of the discrete Josephson logic circuits. The discrete circuits can be conveniently and easily linked to each other, in a practical fabrication of a Josephson network.
A resistive-coupled fluxon divider consisting of Josephson transmission lines is designed and fabricated. Dynamic behavior of the divider is measured using two Josephson samplers connected to both output transmission lines, and successful operations are observed. It is also clarified that whether or not a fluxon signal can be transmitted to each output transmission line is determined mainly by the value of the bias current applied to each such line. Numerical simulations are made, and the results are in good agreement with the experimental ones.
We study the performances of a type of single-phase shift register based on flux-quanta dynamics. The system employs as a clock signal a spatially localized sinusoidal current injection in a discrete Josephson transmission line. This drive generates fluxon–antifluxon pairs which, traveling along the transmission line, move the single flux quanta stored in attractive potential wells. These wells are obtained by inserting inductive discontinuities in the transmission line. The simulations show that the device could operate in real Josephson systems with clock frequencies in the range 10–100 GHz. Good margins are found for the most critical parameters of operation.