One Electron, Two Electrons...

Researchers at the Commerce Department’s National Institute of Standards and Technology will announce in tomorrow’s issue of Science magazine that they have developed the first working prototype of a new standard for measuring the storage of electrical energy—known as capacitance—based on a means of manipulating and counting electrons one at a time. The new system should be much simpler to use and faster than the present technology used to set a recognized standard for capacitance.

Plans call for the standard to be refined so that a final version can be validated by standards organizations around the world and then adopted for use by the electronics and instrumentation industries. The military also is expected to be a big user.

The new standard is the latest to rely on a fundamental property of nature and not be determined by a constructed artifact, classical physics or a combination of both. Nature-based standards currently in use include length (the meter) and time (the second), both defined by precise measurements of the vibrations of the cesium atom.

Electrons are the negative-charged particles that orbit the positive-charged nucleus in an atom. Electrical current is made up of a river of moving electrons. An extremely accurate measure of how many electrons—and therefore, how much electrical power—can be stored in a device (such as a capacitor) is critically important to those developing, manufacturing or using such devices.

The work on the capacitance standard depends on two technologies developed at NIST. The first is an electron pump, based on ultra-small electrical devices called tunnel junctions (thin metal-insulator-metal sandwiches through which electrons can flow). Operating at temperatures less than one-tenth of a degree above absolute zero (approximately 0.1 Kelvin or minus 273 degrees Celsius), the pump passes and counts individual electrons with an uncertainty of 0.01 parts per million. In other words, the pump would miss tallying only one electron in every 100 million passing through.

The second NIST-developed technology is a cryogenic (super-cold), vacuum-gap capacitor. The capacitor is designed so that very few of the electrons placed upon it "leak out," dramatically improving the accuracy of the capacitance measurement. In addition, it does not contain the dielectric materials (which are good but imperfect electrical insulators) that lessen the effectiveness of ordinary capacitors.

The capacitance measurement is made after the cryogenic pump places about 100 million electrons on the capacitor. The resulting voltage generated across the capacitor is determined, and the capacitance is then calculated as the ratio of pumped charge to measured voltage.

NIST’s present primary capacitance standard has been measured to an uncertainty level of 0.01 parts per million. Although very accurate, the measurement requires complex calculations, takes months to complete and is best done at a national measurement laboratory such as NIST. The system for the prototype standard, the researchers believe, will be simpler to use, should make measurements faster and could be set up at any site with the necessary equipment. They add that the new standard would only need to be one-tenth as accurate as the current version to be considered a practical tool.

"Although the engineering challenges involved are substantial, our present understanding of the requirements for reliable operation of [these] devices indicates that they can be met," the authors state in the Science article.

The researchers conclude with the hope that their new capacitance standard will join the two other widely adopted natural electrical standards for voltage and resistance (the opposition that a substance offers to the flow of electric current).

Authors of the paper include Mark W. Keller, Ali L. Eichenberger and John Martinis of NIST’s Electromagnetic Technology Division, Boulder, Colo., and Neil M. Zimmerman of NIST’s Electricity Division, Gaithersburg, Md.