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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

Realisation of the SI second

The International System of Units, SI (Système International), introduced in 1960 by the Metre Convention, lays down the definitions of the physical units. Since 1970, use of the SI units has also been prescribed in Germany by the Units in Metrology Act. The PTB has been entrusted with the realization and dissemination of the SI units. The second is one of the so-called SI base units.

Since 1967, the second has been defined via the resonance frequency between selected energy levels of the non-radioactive isotope 133 of the element caesium,133Cs. The definition followed the development of atomic physics and quantum mechanics and the demonstration of the first caesium atomic clock in 1955.

Primary atomic clocks differ from other atomic clocks in that the rate determining factors are known with extraordinary accuracy. In primary clocks the SI second (“atomic second”) is therefore realised with small uncertainty, and this uncertainty can be estimated on a sound basis. Four primary clocks are currently operated at PTB. The most accurate clocks are the two fountain clocks CSF1 and CSF2. With a probability of 67% the seconds generated by the fountain clocks deviate from the ideal SI second by less than ±2.10‑16. That means that the fountain clocks deviate from an ideal clock by not more than 20 picosecond per day or, in other words, by one second in 130 million years. The primary clocks CS1 and CS2 with thermal atom beam were the most accurate clocks in the world for many years until the nineties with an uncertainty of 1 second in 3 million years.