Performance of the PTB reconstructed primary clock CS1 and an estimate of its current uncertainty

The paper describes the current performance of the primary clock CS1 of the Physikalisch-Technische Bundesanstalt (PTB). The clock employs a thermal atomic beam and magnetic state selection. After major reconstruction during 1995 and 1996, routine clock operation was restarted in May 1997. Results of comparisons with UTC(PTB) and other internationally recognized time scales are presented. An evaluation of the CS1 Type B uncertainty yielded 7 × 10-15 (1σ). The CS1 is thus the most accurate atomic clock in continuous operation world-wide.

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