Quantum bell rings to electron beat

Before the invention of the transistor, telephone switching stations clicked with the sound of small metal arms gating the flow of electricity. A new invention revives mechanical systems, but on a far smaller scale.

The quantum bell: A clapper (C) is set in motion by voltage applied across gates (G) on either side. The clapper is mechanically connected to an island that shuttles electrons between two plates. Erbe et al./PRL

German researchers have come up with a device that takes control of electricity by shuttling electrons across a gap with metronome-like regularity. The mechanism has potential uses in electronic odor detectors and extremely accurate current gauges. A team including Artur Erbe and Robert H. Blick of the Ludwig-Maximilians University in Munich reports its findings in the Aug. 27 Physical Review Letters.

The researchers gave the silicon-containing device an apt name: the quantum bell. Its centerpiece is a clapper that oscillates at 100 megahertz when voltage is applied to either side. Attached to its tip is an island of metal electrically isolated from the clapper. As the island bounces back and forth, it transfers electrons between two plates.

“They use the mechanical motion of the clapper sort of like an [electron] turnstile,” explains Andrew Cleland at University of California, Santa Barbara. By cooling the apparatus to 4.2 kelvins, the researchers came close to their goal of reproducibly moving one electron with each clapper oscillation.

If such one-electron precision is attained, it could lead to an extremely accurate way to tally an electron flow, Cleland speculates.

By cooling the device further or changing its blueprint, the researchers will reach that goal, they predict. “This paper describes probably the most difficult steps toward achieving that,” says Cleland. He says it remains to be seen whether this approach can ultimately perform better than the transistors presently used to measure current and calibrate electronic circuits.

Erbe imagines that the quantum bell could have practical applications at room temperature. For example, if airborne molecules stick to the clapper, they may slow down the oscillation frequency and thus the flow of electrons. That could form the basis for a mechanical nose, he says.

Erbe adds that devices based on the quantum bell, with its high-frequency clapper, could ultimately be used to miniaturize communication tools, including cell phones and radios. Such devices use filters to convert the low frequency of the human voice into high-frequency waves. A quantum-bellbased device would be smaller than current filters and would use less electricity.

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