May 1973 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
The Josephson effect was predicted in 1962 by British physicist Brian David Josephson. It postulated the possibility of a resistance-less path for electrical current across an extremely thin insulator sandwiched between two superconductors. Dr. Juri Matisoo, of IBM, is credited with building the first Josephson junction switch in 1967, demonstrating sub-nanosecond switching times. Back in the day, superconducting materials, like graphene, were resources available only to well-funded research establishments like major corporations, universities, and government facilities. Now, anyone with an interest can order both superconductors and graphene from Amazon or eBay. Because of their quantum nature, Josephson junctions are used widely in metrology as base unit standards.
Fastest Electronic Device
Drs. Wilhelm Jutzi (left) and Theodor Mohr, of IBM, check the new device.
Small bounded oval at center of this photomicrograph is a Josephson junction, measuring 1.25 by 3 microns. Broad white areas are 50-ohm transmission lines connecting junction to the external time-measuring circuitry.
In 1962, the English physicist Brian Josephson predicted that an insulator could behave like a superconductor (complete disappearance of electrical resistance at very low temperatures), provided it was thin enough and was sandwiched between two layers of superconductive metals.
Now, in 1973, scientists at the IBM Research Division have used the Josephson effect, combined with other experimental findings, to indicate that current could pass through an ultra-thin insulating barrier between superconductors in two different ways, to produce an electronic switch that can operate in less than 10 trillionths of a second (picoseconds). In performing this switching action, the device requires only about 1/10,000 the power of the best switching transistor.
This tiny power consumption means that the junctions generate very little heat and thus can be packed very closely together. Since an electrical impulse travels about 1 mm in the time that a Josephson junction switches, dense packing is essential to avoid excessive delay as the signal travels from one circuit to the next.
At low current levels and with no magnetic field present, the current passes (or tunnels) through the insulator as if it were a superconductor. There is no voltage drop across the junction. If the current, or an applied magnetic field, is raised above a certain critical level, conduction through the insulating barrier is by the familiar form of electron tunneling similar to that found in a tunnel diode. The two differences - the presence or absence of a voltage drop - represents the 1 or 0 levels of digital computer logic.
The first Josephson junction switcher was reported in 1967 by Juri Matisoo of IBM, who measured a switching speed of less than 800 picoseconds. The higher speed is a result of the decreased size (1.25 by 3.1 microns), higher current density, and the sophisticated instrumentation developed for measuring such short switching intervals.
Posted February 20, 2018