Physicists fire up strontium atomic clock

+/- one second per 200 million years

The smart choice: opportunity from uncertainty

US physicists are rather pleased with themselves having perfected a clock "so accurate it will neither gain nor lose even a second in more than 200 million years", Reuters reports.

The atomic timepiece, developed in the Joint Institute for Laboratory Astrophysics (JILA) - a collaboration between US Commerce Department's National Institute of Standards (NIST) and Technology and the University of Colorado in Boulder - uses "thousands of strontium atoms suspended in grids of laser light" to achieve an accuracy more than twice that the NIST-F1 standard caesium clock (+/- one second per 80 million years).

The clock's advantage over its caesium counterpart is due to its "use of light, which has higher frequencies than the microwaves used in NIST-F1". ScienceDaily elaborates: "Because the frequencies are higher, the clocks divide time into smaller units, offering record precision."

Specifically, the clock uses a few thousand strontium atoms held in a column of around 100 "pancake-shaped" traps forming the "optical lattice", which is created by "standing waves of intense near-infrared laser light". This is described as "a sort of artificial crystal of light" which "constrains atom motion and reduces systematic errors that occur in clocks that use moving balls of atoms, such as NIST-F1".

Having trapped their atoms, the scientists detect strontium's 430 trillion per second "ticks" by "bathing the atoms in very stable red laser light at the exact frequency that prompts jumps between two electronic energy levels". To improve accuracy, the team managed to "cancel out the atoms' internal sensitivity to external magnetic fields, which otherwise degrade clock accuracy", and also "characterised more precisely the effects of confining atoms in the lattice".

To measure the device's accuracy, the researchers compared its performance to another of their experimental clocks - a similarly optical version using calcium atoms*. ScienceDaily clarifies: "The best clocks can be precisely evaluated by comparing them to other nearby clocks with similar performance; very long-distance [output] signal transfer, such as by satellite, is too unstable for practical, reliable comparisons of the new generation of clocks. In the latest experiment, signals from the two clocks were compared via a 3.5-kilometer underground fiber-optic cable."

Project leader Jun Ye said: "This is our first comparison to another optical atomic clock. As of now, Boulder is in a very unique position. We have all the ingredients, including multiple optical clocks and the fiber-optic link, working so well. Without a single one of these components, these measurements would not be possible. It's all coming together at this moment in time."

Ye's next move is to run the strontium device against the world's most accurate clock, NIST's experimental single mercury ion clock, "accurate to about one second in 400 million years" when fired up back in 2006 and performing "even better today", according to Jim Bergquist, the NIST physicist who built it.

The advantage of using mutiple atoms over the single ion approach is that the former produces a stronger signal, which may eventually allow clocks which outperform even the mercury-based device.

In case you're pondering just where this lust for accuracy is leading, the JILA research is "expected to lead to new scientific tools for quantum simulations that will help scientists better understand how matter and light behave under the strange rules governing the nanoworld", as well as permitting "new tests of fundamental physical laws to increase understanding of the universe".

On a more practical level, a new, super-accurate optical atomic clock will be essential to "synchronize telecom networks and deep-space communications, as well as for navigation and positioning".

While JILA now boasts optical clocks based on aluminium, calcium, mercury, strontium and ytterbium, rival labs worldwide are also working on similar devices and "it is not yet clear which design will emerge as the best and be chosen as the next international standard", ScienceDaily concludes.

The JILA research is published in the latest issue of Science.


*The NIST calcium clock, which relies on "the ticking of clouds of millions of calcium atoms", delivers "high stability for short times", making it not much use as a long-term atomic timepiece, but ideal for comparative purposes.

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