Atomic clock achieves record stability, holds promise for tech

<i>This post has been corrected. See the note below for details.</i>

Atomic clocks built at the official U.S. timekeeping laboratory tick with record-breaking regularity, scientists said — marking an advance that may someday allow researchers to perform new tests of the laws of physics and engineers to perfect technologies such as GPS systems.

The ytterbium optical lattice clocks at the National Institute of Standards and Technology in Boulder, Colo., achieved a so-called stability of one part in 1018. In plain English, that means that “if a clock had been running since the Big Bang, by now it would only be off by one second,” said Vladan Vuletic, a physicist at MIT who was not involved in the work.

Atomic clocks already serve a number of practical purposes: standards bodies like the NIST depend on cesium-based clocks to set international definitions of the second and the hertz (a measure of frequency), and global positioning systems use cesium or rubidium clocks for calibration purposes.


But before the record-setting ytterbium clocks can be put to wider use, researchers will have to show that they are accurate as well as stable, said Andrew Ludlow, a physicist at the NIST and lead author of a study describing a study describing the achievement that was published Thursday in the journal Science. That will involve evaluating uncertainty in the clocks: understanding exactly how various influences change the ticking rate of the atoms — such as gravity, magnetic field, and temperature.

“At that point, we’ll have both ingredients for measuring time,” Ludlow said.

Since the first accurate atomic clocks were invented in the mid-1950s, scientists have worked on improving the technology. For many years, most atomic clocks used atoms excited by microwave signals. The precise frequency of the microwaves serves as a “standard tick” for timekeeping.

So-called optical clocks, like the timepieces Ludlow and his colleagues have been working with at the NIST, use lasers instead of a microwave signal. The lasers have a higher frequency than the microwave signals in the older atomic clocks, which means that the optical clocks tick more rapidly. The increased frequency of the ticks makes the clocks more useful, Ludlow said — much as a yardstick that measures down to sixteenths of an inch offers more precision than one that only indicates feet.

To set the new stability record, Ludlow’s team measured the exquisite regularity of their clocks — which shoot lasers at a lattice of 10,000 neutral atoms, obtaining a sort of average ticking rate across the group — by comparing two clocks’ ticking rates to each other.

MIT’s Vuletic said the group’s achievement was notable because it happened so quickly.

Optical lattice clocks have only been in use for about a decade, and as recently as five to seven years ago their performance lagged behind that of the best microwave-based clocks, said Tom O’Brian, chief of the Time and Frequency division at the NIST.

O’Brian, who was not a coauthor on the Science paper, said the huge improvements in the clocks’ stability was a result of improvements to the lasers in the systems — as well as a better understanding of atomic physics among scientists, which allows clock designers to tune the interactions between atoms in the lattices and obtain better performance.


Assuming that the team can also establish high accuracy in the clocks — and Ludlow said he thought it would — scientists will put the timepieces to work. Someday, optical lattice clocks could become the new international standard, or could help industry build GPS systems that can rapidly pinpoint locations with sub-centimeter-scale precision.

By examining the subtle effects of gravity, magnetic fields, temperature and other influences on the clocks, users might be able to “turn the clocks around” — look at changes in their tick rates to describe gravitational fields in small areas, for instance. Such tools might be useful to a geologist mapping out an oil field or to a climate scientist tracking a large sheet of ice.

“As the clocks get better and better, the number of things you can do with them increases,” said Ludlow.

The next generation of atomic clocks may also be useful to scientists working to test the laws of physics, he added. Atomic clocks have already been used at the NIST to test Einstein’s theory of general relativity (it held up).

In the future, Vuletic said, physicists might even use the devices to test, via lab experiments, whether fundamental constants in physics are indeed unchanging. (Some researchers have expressed doubts.)

“It would be revolutionary if you could show that the speed of light isn’t a constant,” he said.


[For the Record, 12:35 p.m. PST Aug. 23, 2013: An earlier version of this online post said that the ytterbium clocks’ stability was one part in 10-18. The correct measure is one part in 1018. The earlier version also stated that standard microwave atomic clocks have always used a single-ion technology. In fact, single-ion clocks are experimental. The previous version also stated that future GPS systems will be able to pinpoint locations at a centimeter scale. Such systems may be able to pinpoint locations at a sub-centimeter scale, and at a more rapid speed than is possible today.]

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