are part of a circuit, and since the insulating film breaks the connection, no current flows.
However, if the film is thin enough, the quantum-wave functions of electron pairs (so-called Cooper pairs, which are a product of supercooling and are responsible for superconductivity)
on either side of the film overlap
. If the overlap is large enough (i.e., the film is thin enough), the Cooper pairs tunnel through the insulator, creating a current flow. This phenomenon is known as the Josephson effect.
Experiments showed that the current flow is extremely sensitive to magnetic or electric fields because they interfered with the tunneling effect. This interference turns the superconducting circuit on or off within a few picoseconds, making such a junction a very fast switch. Thus, Josephson junctions held out the promise of making ultrafast logic gates.
But making practical versions of these junctions turned out to be tough. There were huge fabrication problems: Circuit traces simply tore apart as the logic gates contracted when cooled in a bath of liquid helium. The junction's switching speed also failed to live up to expectations. Combined with the huge operating expense of chilling them to a few degrees above absolute zero (-273C), this made producing proce
ssors out of these gates unfeasible.
Scientists performed further research when new high-temperature superconductors (these materials became superconducting when cooled to only -196C, the temperature of liquid nitrogen) came to light in the late 1980s. However, at these higher temperatures, thermal noise degraded the sensitivity of the device.
While Josephson junctions didn't realize their potential as computer logic gates, their sensitivity to electromagnetic fields makes them an ideal sensor. In the medical field, for example, Josephson junctions perform real-time measurements of brain activity and monitor the currents that control a heart's muscle activity. These noninvasive measurements of organs allow doctors to isolate brain tumors or recognize cardiac problems without resorting to dangerous and expensive surgery.
photo_link (8 Kbytes)
Whatever Happened To Josephson Junctions?
