Atomic time boffins build better second-watcher

NPL and precision on the final frontier

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Black-body radiation,gravity, and other pesky problems

The caesium atoms inside the clock, though, do get bothered: up to 12 forces from outside create "statistical noise". The same forces affect the other five clocks, too.

These forces include black-body radiation, cold collisions, lasers shining on the wrong atoms, and both the earth's magnetic field and gravity. Take gravity, for example. To measure a second at NPL in London requires a different calibration to the NIST Atomic Clock in Fort Collins, Colorado, to compensate for the effects of gravity. NPL is at an elevation of 79 feet above sea level and suffers more from the effects of gravity on its results than Fort Collins, which is more than 3,315 ft, above sea level in the Rocky Mountains in Boulder Colorado, where the effect of gravity is less.

The hardest factor to eliminate, however, is not gravity: it's that Doppler-like distributed cavity phase the third generation of NPL's clock hopes to mitigate further. Measuring the flip of caesium atoms in the clock is made difficult because the atoms and the microwaves are both moving - hence Doppler effect.

By building a better cavity in NPL-CSF3, the physicists hope they can gain more control over the flow of the atoms for a more precise reading. Gibble's work proposes changes in the geometry of the cavity to reduce further the Doppler effect.

CSF2's cavity has an internal diameter of 4.3cm and a height of 4.3cm, while the component's walls are about one centimetre thick. The external dimensions are larger. Szymaniec reckons NPL will probably approach Gibble to model the new cavity's heat field. "If we are really crazy about reducing this effect we'd follow the design he's proposing," Krzysztof says.

The shape of the new cavity will differ to the one. While still cylindrical, the new cavity will feature staggered end cups to help avoid large phase gradients near sharp edges. Also the microwave energy will be fed to the cavity through four feeds, instead of the current two, to improve the phase distribution uniformity.

NPL-CSF3 isn't the end of work to finally nail the second, however, and NPL, along with NIST in Boulder, the Physikalisch-Technische Bundesanstalt in Germany, SYRTE in Paris and the University of Tokyo are now working on a generation of optical clocks using lasers as the source of radiation rather than microwaves. Laser frequencies are higher so there the errors are smaller and statistical noise is reduced.

"That's in the future; early experiments are promising," Krzysztof told us.

What NPL's man called the "lowest uncertainties" in the results taken from optical clocks have been found in the measurements taken by NIST.

Changing elements

The move to lasers will likely mean the end of the atomic clock as Essen conceived and built it because the optical clocks will need to move off caesium. The reason is that this chemical element doesn't stay excited for long enough using lasers for the physicists to get an accurate reading.

"We'd never have the time to measure the frequency," Krzysztof said.

The optical clocks measure the second either using a process of transitions in single atoms trapped in electromagnetic traps or large numbers of neutral atoms held in traps formed by interfering laser beams – called optical lattices.

The most popular substitutes for caesium so far? Strontium, Ytterbium, and mercury, although NIST uses positively charged aluminium ions – or more precisely AI+ – aluminium with one electron removed to make readings easier.

Szymaniec cautions that while results from optical clocks are encouraging, they are preliminary and such clocks remain in the future. Ahead of that, the third-generation of Essen's offspring is limbering just as humans prepare to depend on it more. ®

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