Uncertainty evaluation of the atomic caesium fountain CSF1 of the PTB

A new primary frequency standard, the atomic caesium fountain CSF1, has been put into operation at the Physikalisch-Technische Bundesanstalt (PTB). 133 Cs atoms are cooled in a magneto-optical trap and in optical molasses. They are launched to a height of 39 cm above the microwave cavity centre. The resulting Ramsey resonance signal has a full-width half-maximum linewidth (FWHM) of 0.88 Hz. The first uncertainty evaluation yields a relative 1 σ frequency uncertainty of 1.4 × 10 −15 . The short-term relative frequency instability of the CSF1 for averaging time τ is typically 3.5 × 10 −13 (τ/s) −1/2, dictated by the available quartz oscillator as the local frequency source.