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Highly precise comparisons of fountain frequency standards

PTB and the US National Institute of Standards (NIST) have carried out a joint project to compare two laser-cooled caesium clocks over a period of several weeks. The measured relative deviation between the two clocks was lower than 0.5 · 10–15, significantly lower than the estimated uncertainty of 1.5 · 10–15 for each frequency standards. The relative deviation was determined with a very small measurement uncertainty of 0.6 · 10–15.

Fountain frequency standard: NIST-F1

Today the most accurate atomic clocks, also called caesium fountains, utilize laser-cooled caesium atoms. PTB operates such a clock CSF1 since 1999. The well defined and low velocity of the atoms yields a stability and accuracy at least one order of magnitude higher than what was formerly achievable with caesium atomic clocks using thermal atomic beams. The stability of CSF1 had already been confirmed by comparison with an active PTB hydrogen maser. Then, during a two month operational period PTB’s CSF1 and the caesium fountain, NIST-F1, at NIST were compared. The two clocks are of similar type and accuracy although they were constructed following different design principles.

First, NIST-F1 and CSF1 were compared with active hydrogen masers available at the respective locations. Then the two masers were compared with each other by two different methods. One method recorded the travelling time of time signals exchanged between the two institutes via telecommunication satellites. The other method was a further development of the well-established technique to register the arrival times of signals from the Global Positioning System (GPS), which has been applied for many years. The results obtained by both methods over the period of the project differed by 0.3 · 10–15. This value was taken into account in the measurement uncertainty stated above. The deviation found does not indicate a systematic difference between the two fountains. Both fountains are thus very accurate at measuring time. The small measurement uncertainty is remarkable inasmuch as the two fountains are operating 9000 km apart from each other.

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