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Optical clock makes leap in precision

24 Nov 2004

UK researchers make strides in developing the world's most precise time reference.

Scientists at the UK's National Physical Laboratory (NPL) have succeeded in making the most accurate optical frequency measurement to date (Science 306 1355). The development is an important step towards constructing optical clocks that are hundreds of times more precise than the world's best timekeeper -- the caesium (Cs) atomic clock. The latter operates at a microwave frequency and is used to define the second.

The NPL measurement was made on the red 5s 2S1/2 -- 2D5/2 transition (674 nm) of a single cooled strontium (Sr88+) ion . The frequency of the transition was measured to an accuracy of 3.4 parts in 1015, a significant improvement over previous similar research.

"This is a factor of three more accurate than the best previously reported optical frequency measurements -- the mercury ion optical frequency standard at NIST and the ytterbium ion optical frequency standard at PTB," Helen Margolis from NPL told Optics.org. "We are confident that with improvements to our setup we should be able to demonstrate that our strontium-ion optical-frequency standard is both more stable and more reproducible than Cs clocks."

Although the NPL results are not yet as good as Cs atomic clocks (which boast a superior stability of just 1 part in 1015 at a frequency of 9.192 GHz) it is thought that optical frequency standards have the potential to far outperform Cs-based microwave standards. Ultimately, an optimized optical clock could result in a new, more accurate way to define the second.

"It has long been speculated that optical frequencies, which are about 100,000 times higher than the Cs microwave frequency, would provide a better method of defining the second because they divide time into smaller slices," explained Margolis. "It has been suggested by a number of research groups that ultimately optical frequency standards could prove to be between a factor of 100 and 1000 better than Cs clocks."

To perform the experiment, Margolis and her colleagues had to capture a single Sr ion in a trap and cool it to just a few millidegrees above absolute zero (-273 °C). It was then probed with a laser beam from an extended-cavity red diode laser that was tuned to search for the transition. An optical frequency comb generated by a femtosecond laser was used to calibrate the diode laser's exact emission frequency when it matched the transition.

The NPL team is now working on refining the setup in order to further improve the accuracy. "Probably the most important improvement is to reduce the linewidth of the laser which we use to probe the 674 nm clock transition," said Margolis. "We are also currently building a second strontium ion trap which will enable us to assess the reproducibility by direct comparisions between the two standards."

Oliver Graydon is editor of Optics.org and Opto & Laser Europe magazine.

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