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En route to an optical atomic clock

“Optical clocks” are considered as candidates for a time standard with higher stability and accuracy. Compared to the best time standard at present, – the Cesium microwave atomic clock –, uncertainties of 10–15 can be reached with clearly shorter averaging times. PTB has achieved a milestone en route to an all-optical atomic clock using Calcium atoms, cooled down to 10 µK, and a mode-locked femtosecond laser frequency comb.

Fluorescence light from a cloud of Calcium atoms during the first stage of laser cooling.

Cooling is achieved in two steps: at first Calcium atoms are decelerated in an ultrahigh vacuum environment by resonant laser light scattering and stored in a magneto-optical trap. Then, the 657 nm transition itself (the “clock” transition) is used for cooling and to prepare a cloud of some 107 ultra-cold atoms at 10 µK. To probe the narrow line (natural line width 0.3 kHz) all laser and magnetic fields are turned off and as the atoms following gravity they are subjected to several flashes of a narrow-band laser with a line width of roughly one Hz. After determining the fraction of excited atoms the laser frequency is tuned to precisely match the resonance frequency of the atoms. Utilizing the new technology of optical frequency combs the frequency of this resonance is readily determined and traced back to the microwave frequency of the primary standard for time and frequency, the Cesium atomic clock.

With ultracold atoms a detection method can be employed which has the capability of reaching a quantum limited noise level. As the atoms in the cloud are probed simultaneously, deviations of the laser from the atomic transition frequency can be detected at very low noise levels. Thus, as a parti-cular advantage of this optical clock, a relative uncertainty of 10-15 can be achieved by averaging over less than one second rather than over several hours (as is the case for Cesium clocks). In principle, the averaging time can be reduced to one tenth of a second.

Contact at PTB:

Phone: +49-531-592-0