Terrific timekeeper: optical atomic clock beats world standard.

Physicists in Colorado say that they've refined an innovative atomic clock to be more precise than the breed of clocks that's been the best for 50 years.

The advance indicates that the reign of atomic clocks tuned to the element cesium is coming to an end, says physicist James C. Bergquist of the National Institute of Standards and Technology (NIST) in Boulder, Colo., who led the work.

To track time, a cesium clock exploits the absorption of microwaves by a cloud of cesium atoms (SN: 9/4/04, p. 150). In contrast, the NIST optical clock makes use of interactions between ultraviolet radiation and a single mercury ion. Ultraviolet electromagnetic waves oscillate about 100,000 times as fast as the cesium-cloud microwaves do and so provide a much finer means to measure a second.

Bergquist and his NIST coauthors describe the tweaks to their clock in the July 14 Physical Review Letters.

"It's a very impressive paper. It basically says that these things [optical atomic clocks] are now capable of being better than cesium [clocks]," says atomic clock specialist Patrick Gill of the National Physical Laboratory in Teddington, England.

Fritz Riehle of PTB, the national-standards lab in Braunschweig, Germany, says that such superiority had already been demonstrated in a comparison of two ytterbium optical clocks at his lab. Still, the NIST results are "a breakthrough," he adds, because the mercury clock's uncertainty of measurement is so tiny.

With further improvements since they submitted their new report, the NIST researchers have made a clock that's about 10 times as precise as the world's cesium standard, Bergquist says. According to NIST figures, the cesium standard would be off by no more than 1 second in 70 million years of continuous operation.

The NIST advance could ultimately improve navigation and telecommunications systems, says Jean-Jacques Zondy of France's National Metrology Institute in La Plaine Saint Denis. Beyond that, the achievement "raises the issue of changing the definition of time," he notes.

However, a redefinition of the second, now based on a specific property of cesium, may be decades away, says Zondy. Scientists don't yet know whether some other atom will prove better than mercury in optical clocks.

The record-low uncertainty of the NIST clock opens the door to ultraprecise tests of foundations of physics, including relativity and the steadiness of the so-called fundamental constants (SN: 5/14/05, p. 318).

At least one such test is already well under way, Bergquist says. In recent years, astrophysical data have indicated that the fine-structure constant called alpha has increased since the early universe. By comparing the behavior of the new mercury clock and another NIST optical clock based on aluminum, the NIST team is seeking evidence that alpha may be changing today.