Improvements of the systematic uncertainty, frequency instability, and long-term reliability of the two caesium fountain primary frequency standards CSF1 and CSF2 at PTB (Physikalisch-Technische Bundesanstalt) are described. We have further investigated many of the systematic effects and made a number of modifications of the fountains. With an optically stabilized microwave oscillator, the quantum projection noise limited frequency instabilities are improved to 7.2 × 10 −14 (τ /1 s) −1/2 for CSF1 and 2.5 × 10 −14 (τ /1 s) −1/2 for CSF2 at high atom density. The systematic uncertainties of CSF1 and CSF2 are reduced to 2.74 × 10 −16 and 1.71 × 10 −16 , respectively. Both fountain clocks regularly calibrate the scale unit of International Atomic Time (TAI) and the local realization of Coordinated Universal Time, UTC(PTB), and serve as references to measure the frequencies of local and remote optical frequency standards.
We present experimental work for improved atom loading in the optical molasses of a caesium fountain clock, employing a low-velocity intense source of atoms (LVIS) [Lu et al., Phys. Rev. Lett. 77, 3331 (1996)], which we modified by adding a "dark" state pump laser. With this modification the atom source has a mean flux of 4 × 10 8 atoms/s at a mean atom velocity of 8.6 m/s. Compared to fountain operation using background gas loading, we achieved a significant increase of the loaded and detected atom number by a factor of 40. Operating the fountain clock with a total number of detected atoms Nat = 2.9 × 10 6 in the quantum projection noise-limited regime, a frequency instability σy (1s) = 2.7 × 10 −14 was demonstrated.
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