How Cool Is That?

AUSTIN, TEXAS--A person who has one thermometer knows the temperature, physicists sometimes quip, but someone who has two is never sure. That adage may not remain true for long, however, now that researchers have developed a type of electronic thermometer that needs no calibration.

A thermometer is only as accurate as its scale, which is set by comparison to better thermometers and ultimately to several natural reference points. Researchers with the National Institute of Standards and Technology (NIST) use the point at which water ice and steam can peacefully coexist--273.16 kelvin--and other references to calibrate thermometers made of platinum wire, whose resistance decreases as the platinum gets colder. Thermometer manufacturers then use these platinum-resistance standards to set the scales for other high-precision thermometers. If two lab thermometers disagree, it's impossible to tell which is right without resorting to the standards again.

But a tiny gadget called a tunnel junction could change that, say Lafe Spietz, Robert Schoelkopf, and colleagues at Yale University. The junction consists of two bits of aluminum sandwiching a layer of aluminum oxide only a few atoms thick. Electrons cannot pass through the oxide freely but must burrow through it one by one. As they do, the current jitters up and down. This jitter is called "shot noise," and it increases in a precisely predictable way as researchers ramp up the voltage across the junction. Moreover, the way the noise changes with voltage depends only on the temperature and fundamental constants such as the charge of the electron, Spietz told a meeting of the American Physical Society here on 5 March. And that means researchers can determine the exact temperature by simply tracing the variation of the shot noise. Indeed, the researchers showed that the device could track temperature from less than a degree above absolute zero to room temperature.

The new thermometer might someday replace the elaborate collection of reference points, says Samuel Benz of NIST in Boulder, Colorado, who is working on an electronic standard for higher temperatures. Before that happens, Benz says, the researchers "need to push down the uncertainties." The device is accurate to within 0.1% in the range from less than a degree above absolute zero to nearly room temperature, but the researchers think they can make it many times more accurate.

Related sites
Robert Schoelkopf's home page
The NIST Thermometry Group