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Thursday, 9 March, 2000, 15:35 GMT
Smallest circuits show quantum effects
The smallest electrical circuits ever made
By BBC News Online science editor Dr David Whitehouse

Physicists have confined electrons to the smallest circuits ever made and witnessed their strange behaviour.

The scientists say the microscopic devices could one day play an important role in the design of so-called quantum computers which could be far more powerful and smaller than today's machines.

Quantum means "lump" and is the basis of a theory that deals with matter and energy coming in discrete quantities. In the main, quantum effects only show themselves at the atomic level and smaller. Quantum theory shows us that the world of the really small obeys rules that in our experience seem illogical, like sub-atomic particles being in two places at the same time.

Usually, an electron with no excess energy will not care which direction it points in space. But adding a magnetic field can change that. An electron confined to a wire ring in a magnetic field zips around at high speed in only one direction.

Quantum dots

An experiment to verify this is described in the current issue of the Physical Review Letters. It is authored by Axel Lorke at the Ludwig Maximilians University in Munich, and collaborators at the University of California in Santa Barbara (UCSB).

The experiment used the smallest rings ever made to support measurable currents. Only 50 billionths of a metre across, the rings displayed strong quantum effects when they contained only one or two electrons.

These small rings are called quantum dots. They are microscopic regions of a semiconductor surface, which allow researchers to study the behaviour of electrons in a new and controlled way.

Some scientists hope that quantum dots may be used as the basis for memory in the quantum computers of the future. Using a technique that involved spraying two atomic layers onto a semiconductor surface, the researchers made as many as 1011 quantum dots.

Ground state energy

The scientists placed an array of quantum rings between what is essentially a pair of metallic plates and increased the voltage between them until exactly one electron jumped across.

That voltage gave the researchers data about the electron's ground state energy, which increased predictably as the team increased the magnetic field piercing the rings.

But when the field reached a certain threshold, a sudden change in the ground state energy indicated that the electrons had started pointing a particular direction.

What this means is that the quantum rings allow one or two electrons to circulate in specific quantum states giving scientists what they say is a "clean" system for studying quantum behaviour.

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08 Feb 00 |  Sci/Tech
Electronic circuits scale down
20 Jan 00 |  Sci/Tech
Scientists probe the quantum world
22 Oct 99 |  Sci/Tech
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