Working Group 4.41
Unit of Time

Realisation of the SI second

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 operating parameters determining their rate are known with extraordinary accuracy. The atomic second is then realised with small uncertainty, and this uncertainty can be estimated on a sound basis. Five primary clocks have to date been set up at PTB. It is one aim of our group to further reduce the uncertainty of the realisation of the SI second. For many years, CS1 and CS2 were the most accurate clocks in the world. With a probability of 67 % the seconds generated by CS2 deviate from "ideal" atomic seconds by less than ±1.2^.10^-14. This means that CS2 deviates from an ideal clock by not more than 1 nanosecond per day or, in other words, by one second in 2.5 million years. PTB's most accurate clock is a so-called caesium fountain: Its uncertainty is smaller by a factor of 10 compared to CS2.

More information on SI second and atomic clocks
 

• History and present definition of the unit of time
• Scheme of operation and typical realisations of atomic clocks
• The primary clocks CS1, CS2, and CS3 of PTB
• Scheme of operation of a fountain clock
• The fountain clock CSF1 of PTB