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Progress with fountain clocks

PTB-News 3.2019
Especially interesting for

metrology of time and frequency

fundamental research in physics

PTB's two caesium fountain clocks, CSF1 and CSF2, have been established as primary frequency standards and as the basis of PTB᾽s time scale for a long time. Exhaustive investigations have now allowed the uncertainties of these two clocks to be reduced by a factor of 5 (CSF1) and 2.4 (CSF2). With relative uncertainties of 2.7 ⋅ 10–16 reached for CSF1 and 1.7 ⋅ 10–16 for CSF2, these two clocks range among the best in the world besides a few other fountain clocks.

PTB's caesium fountain clocks CSF2 (in the foreground) and CSF1. The atoms are cooled by lasers in the lower part of the vacuum setup and then launched upwards. In the upper part of the setup, they are irradiated with microwaves during their flight.

In caesium atomic clocks, caesium atoms are irradiated with microwaves of a suitable frequency to induce a change in the atomic state. The frequency of the microwave signal is determined by universal properties of the caesium atoms and is the basis of the atomic definition of the unit of time.


Using laser-cooling methods, it had become possible for the first time in the 1990s to form mm3-sized “clouds” of a few millions of caesium atoms with velocities in the range of cm/s and to bring them onto a fountain-like trajectory with a maximum height of approx. 1 m so that they could be irradiated by microwaves. This new geometry allows frequency-shifting effects and the clock᾽s frequency noise to be considerably reduced compared to conventional atomic beam clocks. PTB᾽s caesium fountain clocks CSF1 and CSF2 have been used as primary frequency standards since 2000 and 2008, respectively. Both of them determine legal time that is disseminated by PTB in Germany and contribute to calibrating International Atomic Time (TAI). The CSF2 fountain clock is the national standard of time and frequency.

In addition to their multifaceted use, CSF1 and CSF2 have been subjected to exhaustive investigations of their frequency- shifting effects over the past few years. In this context, particular attention has been paid to effects of the phase of the microwave field and collisions between the caesium atoms. Novel experimental and theoretical investigations of the microwave field effects have been conducted in order to find an operating mode in which the phase variations of the microwave field experienced by the atoms and thus the frequency shifts resulting from these variations are minimized. In the case of CSF1, the collisional shift, and thus also its uncertainty contribution, are minimized by means of skillful manipulations of the motional parameters of the atoms contributing to the clock᾽s signal. CSF2, however, is conceived in such a way that the collisional shift can be determined very accurately by operating the clock alternately at full atomic density and at exactly halved atomic density.

Due to its reduced uncertainty, CSF2 makes the largest contribution to the control of TAI among the whole group of internationally available primary caesium clocks. A comparison has recently been carried out, effectively as an acid test, between the fountain clocks at PTB and those located at LNE-SYRTE (Laboratoire National de métrologie et d'Essais – SYstème de Références Temps-Espace) in Paris via an optical fiber link of 1400 km in length. The results obtained showed excellent agreement between all of the four caesium fountain clocks involved.


Stefan Weyers
Department 4.4
Time and Frequency
Phone: +49 531 592-4410
Opens window for sending emailstefan.weyers(at)ptb.de

Scientific publication

S. Weyers, V. Gerginov, M. Kazda, J. Rahm, B. Lipphardt, G. Dobrev, K. Gibble: Advances in the accuracy, stability, and reliability of the PTB primary fountain clocks. Metrologia 55, 789–805 (2018)