The world's best caesium atomic clocks control Coordinated Universal Time (UTC), an atomic time scale on which the time zones used in everyday life are based. But also in navigation (GPS), astronomy, telecommunications, geodesy and physical fundamental research, accurate timing is of essential importance. Until recently, three of PTB's clocks belonged to the exclusive club of primary caesium atomic clocks. Now a fourth one, the caesium fountain clock CSF2, has joined in.

This month, its data will be taken into account for the first time for the determination of UTC. PTB and the French time-keeping institute are thus the only institutes worldwide which operate four primary atomic clocks. The traditionally large contribution of PTB's clocks to UTC has thus increased considerably.

The determination of UTC is a complex task which the "Time, Frequency and Gra-vimetry" Department of the International Bureau for Weights and Measures (BIPM, Bureau International des Poids et Mesures) dedicates itself to. BIPM takes on the task of providing a worldwide uniform and unambiguous system of measures on the basis of the International System of Units. For the computation of UTC, BIPM relies on the averaging of some 300 atomic clocks in over 60 time-keeping institutes world-wide.

In order to ensure that the UTC seconds correspond as accurately as possible to the definition of the second given in the International System of Units, UTC is con-trolled by a few, particularly accurate, so-called "primary" caesium atomic clocks. In the years 2008 to 2009, this rather small circle of primary clocks consisted of a total of 12 caesium atomic clocks from 7 time-keeping institutes.

Three of these belonged to PTB. This month, the data of a fourth, newly developed, primary PTB atomic clock named CSF2 have been used for the first time for UTC control purposes.

PTB's four primary clocks are the two caesium beam clocks CS1 and CS2, as well as the caesium fountain clocks CSF1 and CSF2. The beam clock CS1 has been in op-eration for 40 years, whereas CS2 has been used as a primary clock for approx. 25 years.

The fountain clock CSF1 has for a few years belonged to the more modern and more accurate clock generation which is, however, now complemented by CSF2. Both fountain clocks already belong to the most reliable and most accurate primary caesium fountain clocks worldwide. Just as its slightly older brother CSF1, CSF2's inaccuracy will not exceed one second in 40 million years.

In fountain clocks, caesium atoms are first strongly cooled by means of laser light, which slows them down to velocities of centimetres per second. The "cloud" of slow caesium atoms which is thus produced is accelerated vertically so that the atoms be-have like water drops in a fountain: after having been tossed, they gradually become slower until they finally fall down after having risen by approx. a metre. During this flight, the atoms are irradiated with microwaves in order to bring them into a different energy state.

Caesium fountains are more accurate than conventional caesium beam clocks espe-cially because the atoms in fountain clocks are slower so that there is more time available to determine the decisive property of the caesium atoms which is necessary for "time generation": their resonance frequency. Only if the frequency of the micro-waves is in agreement with the latter do the atoms change their energy state. Ap-proximately nine billion microwave oscillations have to be completed before exactly one second has passed.

This is the definition as stipulated in the International Sys-tem of Units. Thanks to the availability of two fountain clocks having a similar accu-racy, it is possible for the scientists to further reduce the uncertainties by comparing the two clocks.

Original publication: [1] V. Gerginov, N. Nemitz, S. Weyers, R. Schroder, D. Griebsch, R. Wynands, Un-certainty evaluation of the caesium fountain clock PTB-CSF2, accepted for publica-tion in metrologia (2010)