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High-accuracy optical frequency standard

With the aid of a single stored ytterbium ion, the frequency of blue laser light has been stabilized with an accuracy of a few hertz. Thus, a reference frequency in the optical wavelength region has been established which, at present, is among the most accurate in the world.

The ion trap used in the experiment: A) end cap electrodes and B) ring electrode (1.3 mm diameter) to generate the quadrupole trap field, C) ytterbium oven, D) electron source (heatable filament)

The SI units for time (second) and frequency (hertz) are the measurement units which can be realized by far with the highest precision of any SI unit. Today, PTB and other metrology institutes operate the most precise cesium atomic clocks in the world on a routine basis. With these clocks a relative accuracy of almost 10-15 is achieved for the realization of timing signals and standard frequencies (see PTB-news 01.3). However, precision measurements of fundamental constants as well as new tests of fundamental physical theories require the comparison of precise clocks and call for atomic frequency standards of even higher accuracy and short-time stability. This can be achieved by increasing the frequency of the clock signal with which the reference atoms are excited. In this context the shift from the microwave range (cesium clock: 9,2 GHz) to the higher optical frequency region (100 THz to 1000 THz) is of particular advantage. The accuracy of an optical frequency standard can be transferred to the microwave range and to any other optical frequency using optical frequency comb generators (see PTB-news 00.3).

A single laser-cooled ion stored in a radiofrequency ion trap offers a particularly high accuracy potential for optical frequency standards. The 171Yb+ ion investigated at PTB is one of the ions for which systematic perturbations of the atomic transition frequency seem to be controllable up to a relative uncertainty of 10-18. In the experiments the resonance signal of a stored 171Yb+ ion was used to stabilize the frequency of a specially designed 435,5 nm semiconductor laser system. The laser frequency thus defined in atomic terms was measured using a frequency comb generator and the PTB cesium reference standard CSF1. The statistical uncertainty component dominates the 1-rho-uncertainty of 6 Hz which was achieved so far for the measured frequency value (688 358 979 309 312 Hz) so far achieved. The frequency values measured within several weeks differ by only ± 2,6 Hz.

Very likely the uncertainty of the 171Yb+ frequency standard can be further reduced by several orders of magnitude in future work.
At present an experiment is being prepared to directly compare two 171Yb+ standards. The goal is to obtain information about the influence of the storage conditions on the atomic transition frequency - independent of the accuracy and stability limitations of the cesium reference. Future work will aim at significantly reducing the measurement uncertainty.

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