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Panoramic view of the clock hall at PTB with the four caesium clocks CS1, CS2, CSF1 and CSF2.

Unit of Time

Working Group 4.41


The working group develops and operates primary atomic caesium clocks  as standards of time and frequency, according to the definition of the unit of time “Second” stated within the International System of Units (SI). The ensemble of clocks built at PTB consists of the conventional caesium clocks with thermal atom beam CS1 and CS2 and of the modern caesium fountain clocks CSF1 and CSF2 (German national standard for time and frequency), based on laser-cooled atoms. The latter belong to the world’s most accurate atomic clocks.

One major task is the utilization of these four primary caesium atomic clocks for the realization of the national reference time scale UTC(PTB), in collaboration with the working group Dissemination of Time. Another major task are frequent clock contributions for the steering of International Atomic Time (TAI) by the BIPM (Bureau International des Poids et Mesures), to ensure the worldwide agreement of reference time scales.

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The performance of a primary clock is characterised by the physical quantities frequency stability and systematic frequency uncertainty. On the other hand, it is expressed by its availability time and its reliability. The working group is continually improving the clock performance in all these fields by research and further development. As a result, the fountain clocks CSF1 and CSF2 belong to the world’s foremost atomic clocks, which are a handful of caesium fountain clocks in the major national metrology institutes.

By repeated measurements of absolute optical frequencies in collaboration with the working group Optical Clocks with Trapped Ions the working group 4.41 is involved in research on potential temporal variations of fundamental constants, which are predicted by various theories unifying the fundamental interactions.

An important upcoming project is the scheduled comparison of the PTB fountain clocks with fountain clocks in other countries by making use of the Atomic Clock Ensemble in Space (ACES), which is expected to go on board of the International Space Station (ISS) in the near futureOpens external link in new window.

It is anticipated that this project will achieve an unprecedented accuracy for remote caesium fountain clock comparisons.

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By steering of the national reference time scale UTC(PTB), the primary atomic clocks constitute the basis for PTB’s  time services. Likewise by their TAI steering contributions, the frequency stability and accuracy of International Atomic Time is supported.

Moreover, the fountain clocks of the working group 4.41 offer the possibility of measurements of absolute frequencies at the highest accuracy. The fountain clocks serve, on the one hand, to control other atomic clocks. On the other hand, they provide valuable assistance for the development of future, even more accurate optical clocks. For this purpose absolute measurements of optical transition frequencies, which come into consideration for a future redefinition of the unit of time, are performed by the fountain clocks. This work is done in collaboration with the working groups Optical Clocks with Trapped Ions and Optical Lattice Clocks at PTB.

Beyond that, in collaboration with the working group Frequency Dissemination with Fibres, the PTB fountain clocks were also used for absolute measurements of optical transition frequencies at the Max Planck Institute of Quantum Optics in Garching (DE) and at the Leibniz University in Hanover (DE), by making use of long-haul optical fibre connections.

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