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IEEE Transactions on Magnetics

Real time fluxon dynamics in josephson transmission line

Hiroshi Akoh, Shigeki Sakai, A. Yagi, Hisao Hayakawa

Published Mar 1, 1985• 32 citations

Abstract

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.

DOI: 10.1109/tmag.1985.1063760

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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.

Characteristics of SFQ signal propagation in oxide circuits were investigated by numerical simulation. The numerical simulation for the SFQ signal propagating in the Josephson transmission line (JTL) was performed by assuming the inductor to be a passive transmission line and by taking into account the high I/sub c/R/sub n/ value of oxide junctions. The SFQ signal was found to oscillate when the inductor length was more than 20 /spl mu/m. The origin of the oscillation and the oscillation conditions were investigated with respect to the Josephson plasma frequency. A shunt resistor connected between an inductor and the groundplane was found to be effective for suppressing the oscillation. The JTL in which shunt resistors are connected may also be used as a delay line.

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.