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Quantum dot to turnstile.

Squeezing electrons into specially fabricated, microscopic "boxes" can have a drastic effect on their behavior. At such small dimensions, quantum effects become important, and the confined electrons exhibit a variety of properties unlike those of mobile electrons in a metal. Moreover, these electron boxes, or "quantum dots," are so small that electrical currents through them can no longer be treated as continuous flows of charge.

"You see new effects that people didn't think of before, and you try to understand them," says theorist Yigal Meir of the University of California, Santa Barbara. In addition, "You can check the predictions of quantum mechanics."

Researchers create quantum dots by building semiconductor structures that confine electrons to a volume only a few nanometers across. Because these structures have dimensions comparable to wavelengths typically associated with electrons in semiconductors, electrons trapped in a quantum dot can exist only in a handful of specific quantum states that depend on the shape of the dot's boundaries. In a sense, they start behaving like electrons bound to an atomic nucleus.

The existence of discrete energy levels means that the addition or substraction of even a single electron markedly alters a quantum dot's characteristics energy spectrum. Moreover, the precise number of electrons occupying a quantum dot strongly affects the transport of electrons through the dot. For example, at sufficiently low temperatures (near 1 kelvin), adding one electron to a quantum dot can change the current allowed through by a factor of at least 100. This effect can be used to construct an extremely sensitive transistor capable of switching surprisingly large currents on or off.

. . . to capacitor

Raymond C. Ashoori of AT&T Bell Laboratories in Murray Hill, N.J., and his co-workers have developed an extraordinarily sensitive technique for probing a single quantum dot to observer what distinct and different energy levels an electron can occupy. This new type of spectroscopy relies on the tiny signal generated when single electrons tunnel between a metallic layer and a quantum dot built into gallium arsenide. In effect, the quantum dot acts as a miniature capacitor -- a device for strong electrical energy.

Because electron tunneling into the dot occurs only for particular values of a voltage applied across the device, the researchers can deduce the energy required for adding successive electrons to an initially vacant quantum dot. The sequence of peaks seen at these voltages reflects the quantum dot's electronic spectrum, Ashoori says.

. . . to turnstile

Leo P. Kouwenhoven of the Delft University of Technology in the Netherlands and his collaborators have created an ingenious quantum-dot device that packages electrical current into bundles of a certain number of electrons each. Constructed by putting metal "gates," or barriers, at the entrance and exit to a quantum dot fabricated at the boundary between layers of gallium arsenide and aluminum gallium arsenide, the device acts as a turnstile. An oscillating voltage alternately raises and lowers the barriers so that one is down when the other is up. This permits a certain number of electrons to tunnel into, then out of, the quantum dot during each cycle. By selecting the appropriate frequency, researchers can specify the number of electrons in each bundle.

Such a device for counting electrons may serve as a means of setting a standard for the measurement of electrical current. Thus, an ampere--the fundamental unit of electrical current--could be defined directly as the passage of a certain number of elementary charges per second.
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Title Annotation:American Physical Society report
Author:Peterson, Ivars
Publication:Science News
Date:Apr 4, 1992
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