Circuits which rely on bundles of magnetic energy rather than electrons
have been made to switch hundreds of times faster than equivalent electronic
circuits by researchers in Japan, the USSR and the US. The circuits are
made from superconductors, which lose all their electrical resistance when
cooled to very low temperatures.
The magnetic bundles, known as magnetic flux quanta, are the smallest
possible amount of magnetic energy which can exist in a superconductor,
and their size is related to the magnetic field strength. They are analogous
to electrons, the smallest electric charge possible. Both can carry signals
around a circuit but flux quanta are very unlikely to be seen in ordinary
metals because they are scattered and effectively lost if there is any electrical
resistance. For practical purposes they exist only in superconductors.
Several groups of researchers reported work with single flux quantum
circuits at the International Superconductive Electronics conference in
Glasgow last week. Circuits made at the Institute of Radio Engineering and
Electronics in Moscow have been made to switch 30 billion times a second
and should work 10 times as fast if they were made smaller. That makes them
100 to 1000 times as fast as typical semiconductor chips.
The magnetic circuits are fast partly because they are switching single
quanta, rather than large numbers of electrons. No scattering of quanta
occurs in superconductors, unlike electrons, which are scattered by atoms
in the materials they move through.
Magnetic circuits can be packed close together so that signals have
a smaller distance to travel. The magnetic switches in the circuits consume
very little power – as little as a billionth of a watt – so they do not
have to be spread out to keep cool.
Flux quanta can be transferred around a circuit in a different way from
electrons: they move between loops by induction. A flux quantum in one loop
induces a quantum in the next loop and in the process disappears, so the
quantum seems to ‘hop’ from one loop to the next. Jim Lesurf of the University
of St Andrews says: ‘You need a small loop to see what is happening.’ In
a large loop the single quanta could not be distinguished, and would be
seen simply as a magnetic field.
Single quanta represent information naturally, according to Richard
Humphreys, the leader of the high temperature supercon ductivity team at
the Defence Research Agency in Malvern. A one or a zero can be represented
simply by a quantum or the lack of a quantum. Electronic circuits use arbitrary
voltage levels to represent ones and zeros. Humphreys says that magnetic
logic circuits can operate very fast because the timing of calculations
is not critical. The circuit elements do not need to be told when all the
data required for a calculation is available. Many electronic circuits need
to wait for a signal, called the clock, before operating.
The teams in Moscow, Tokyo and Pittsburgh have already made logic gates,
including ‘and’ and ‘or’ gates, adders, and converters between analogue
and digital signals.