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With what accuracy do PTB's atomic clocks work?

For the comparison of the atomic clocks constructed at PTB, the rate errors are

1 to 3 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). In physical terms, for CS2 from which the legal time is derived, the uncertainty has been estimated at 12 x 10^{–15} (This means that over the course of a year, an error of one millionth of a second must be reckoned with relative to an ideal clock.) With the cesium fountain, an even smaller uncertainty, namely 1 x 10^{–15}, is achieved. It has also been proved that the seconds of two fountains (of NIST/U.S.A. and of PTB) differ by no more than 1 x 10^{–15}.

© Physikalisch-Technische Bundesanstalt, last update: 2012-05-31, Volker Großmann Printview, PDF