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The second


Since the redefinition of the second in the SI in 1967, the primary cesium atomic clocks, which realize the unit of time, the second, have become more exact by approximately one order of magnitude in each decade. Recently, however, the reproducibility and the stability of optical atomic clocks have become superior to the best cesium atomic clocks by orders of magnitude. In the case of these optical atomic clocks – instead of the microwave transition in cesium – a narrow line in the optical spectral range is interrogated with a laser. This opens up the possibility of defining the SI second – in due time – via a suitable optical transition.

Pre-cooled Sr atomic cloud with a temperature of a few millikelvin. After the temperature of the atoms had been reduced by a further factor of 1000 by means of laser cooling with red light, they serve as a reference in the lattice clock of PTB.

At present, there is an exciting race worldwide about the question of which technology will, in the long term, prove to be the method of choice for a redefinition: Single ions which are stored in a radiofrequency trap – or many neutral atoms which are held in an optical lattice against the Earth’s field of gravity. PTB has, therefore, made it its task to investigate both technologies and to develop optical atomic clocks of the highest accuracy which are at the top worldwide and to prepare the redefinition in this way.

Important results of the year 2015 were the evaluation of the Yb+ clock with a fractional systematic uncertainty of 3 · 10–18 and the first comparison of two optical lattice clocks with Sr via a fiber link between Braunschweig and Paris. PTB is only one of a few laboratories in the world which has an ensemble of different optical clocks at its disposal allowing reliable evaluations in the uncertainty range below that of primary cesium clocks to be performed in frequency comparisons.

The frequency ratio of the Yb single ion clock and of the Sr lattice clock has, for example, been repeatedly determined over a period of more than two years, which allows conclusions as to the stability and reproducibility and as to the accuracy of the two clocks to be drawn. At the same time, this measurement also provides a clearly improved limit for a possible drift of fundamental constants which is predicted by most of the tests which combine the general theory of relativity with quantum mechanics. With these optical clocks, PTB has the most accurate measuring instruments at its disposal which allow the fundamental theories to be tested and, thus – possibly – also new physics to be discovered.