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

How accurate are PTB's atomic clocks?

The rate errors of the best atomic clocks constructed at PTB are in the range of 0.02 billionths of a second per day (for comparison: a quartz wrist watch has a monthly inaccuracy of a few seconds, mechanical watches –  no matter how expensive they may be! – are wrong by a multiple of this). This means that over the course of a year, it must be reckoned with an error of 6 billionths of a second relative to an ideal clock. In physical terms, for the fountain clock CSF2 from which the legal time is derived, the uncertainty has been estimated as 2 x 10–16. It has also been proved that the seconds of fountains at the French SYRTE and PTB do not differ by more than a few 1 x 10–16.