U.S. time now flows from atom fountain

Just days before the new year began, the United States followed through on a resolution to keep better track of time in the years ahead. On Dec. 29, the National Institute of Standards and Technology (NIST) in Boulder, Colo., switched to a new type of timekeeping technology—the atomic fountain clock—as the nation’s primary time standard (SN: 8/7/99, p. 92).

Stunning developments in atom cooling and trapping since the 1980s led to the new clock type (SN: 10/25/97, p. 263). The NIST F-1 fountain clock that went into service last month uses lasers to congeal cesium atoms into a cold ball.

As its name suggests, the fountain clock tosses the ball upward, like a water droplet spurted upward from a spout. As the ball rises a meter and then falls under gravity, the instrument adjusts its microwave frequency to maximize the cesium’s excitation. Doing so, it tunes itself to the atoms’ resonant frequency, which defines the second.

Drifting by less than one second per 20 million years, F-1 already gives several-times-better accuracy than its predecessor, NIST-7 (SN: 5/1/93, p. 276). The older clock uses a beam of heated cesium atoms to tune its microwave frequency.

Looking forward to a yet-more-accurate future, NIST’s Steven Jefferts, one of F-1’s builders, says, “I’d be pretty disappointed if we didn’t get another factor of three out of it.”

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