Counting millions of electrons, one by one.

In his "Foundation" series, science-fiction writer Isaac Asimov coined the phrase "I don't give an electron" to mean "I couldn't care less." Researchers at the National Institute of Standards and Technology in Boulder, Colo., however, care about every electron in their pursuit of accurate electrical measurements.

Mark W. Keller and his coworkers at NIST have developed an electron pump that shunts electrons one by one onto a capacitor for storage. They have used the pump to count millions of electrons, missing, on average, only 1 in 70 million, they claim.

"With this device, electron counting has advanced from a novel laboratory phenomenon to a process that is accurate and reliable enough to be the basis of a new metrological standard of capacitance," says the NIST team in a report to be published in Applied Physics Letters.

The NIST electron pump consists of a chain of six microscopic islands of aluminum separated by tiny patches of aluminum oxide. The patches act as tunnel junctions, allowing an electron to pass through only when an electric pulse is applied. A sequence of appropriately timed pulses forces an electron to pass from island to island down the chain to the capacitor. Only after one electron exits does the next electron enter the pump.

Thus, the pump can be used to place an exactly specified number of electrons into storage. "We call it an electron counter, but it's really an electron controller," Keller says. "We're forcing electrons to go through the device in a prescribed way."

"There's no limit on how many electrons we can pump through the device," he adds. The chance of an error, however, increases slowly as the number goes up.

By knowing exactly how many electrons a capacitor holds and carefully measuring its voltage, researchers can get a direct measure of the electric quantity called capacitance, which reflects charge-holding ability.