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Caesium fountain

World-class frequency standards that you can rely on

The caesium fountain primary frequency standard apparatus is used to realise the SI definition of the second and contributes to the construction of the international time scale, UTC.

We can supply caesium fountain primary frequency standards to other national standards laboratories or organisations needing direct reference to a realisation of the SI second, such as time distribution centres and large scientific facilities. The long-term stability, better than 100 ps/day, and accuracy of the standard can be used to correct clocks which form local time scales in national timing laboratories or in academic and industrial institutions. The standard will also be useful to observatories which require precise timing for astronomical observations and satellite laser ranging.

Customers can trust our proven ability to deliver complex measurement systems to organisations, including other national measurement laboratories. Our cost-effective solutions can be used to increase the availability of accurate frequency references and timescales in critical locations.

The design of our commercial caesium (Cs) fountain is based on those we operate at NPL, which contribute to the international time scale, UTC, and provide stability for the UK national timescale, UTC(NPL). It comprises a physics package, lasers and optics for cooling and probing, as well as control electronics. A microwave signal from a local oscillator is tuned to the ground state hyperfine transition of the Cs atoms and the probability of the transition is measured. A small change in the measured probability is a direct measure of the frequency deviation of the local oscillator, which can be corrected accordingly, and the value is recorded for analysis.

Using atoms, which are slowed down by laser light (laser cooling technique) and fly freely under gravity (as in a fountain), greatly increases the interaction time with the microwaves. This enables frequency measurement precision equivalent to less than 1 part in 1013 for one second of averaging time. The long-term accuracy of this fountain standard is at parts in 1016, limited by uncertainty of systematic frequency shifts, such as perturbations by stray external fields.


Key facts and data

Short-term stability <1-3 × 10-13/√τ (depending on the local oscillator used)
Accuracy <5 × 10-16 (subject to evaluations by the user)
Uptime >95%

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