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Fountain atomic clocks become even more precise

Thanks to experiments carried out on PTB's CSF1 fountain atomic clock, a new possibility of compensating a perturbing effect has been found. This leads to a significant improvement of caesium fountain clocks.

The two caesium fountain clocks in the clock hall at PTB currently rank among the most precise clocks in the world.

In the International System of Units (SI), the second is defined on the basis of a specific microwave transition between two internal energy states of the caesium atom. In a caesium fountain clock (the best type of primary atomic clock presently available), laser beams help to trap the caesium atoms in a small cloud and to cool them down; after that, the atomic cloud is thrown approx. one meter upwards and then falls down again. During this free-flight phase, the transition frequency can be determined very accurately. Modern fountain clocks realise the length of the SI second with a relative uncertainty of better than 1 · 10–15.

Crucial for the operation of any primary atomic clock is that all effects which might alter the resonance frequency of the atoms are understood in detail so as to avoid or correct the frequency shifts resulting from them. An essential correction becomes necessary for the caesium fountain clock due to the mutual collisions of the cold atoms in the cloud. In general, the limited accuracy with which this correction can be carried out contributes substantially to the uncertainty of the duration of a second which is derived from a fountain clock.

Now, a new method has been developed by which the collision-induced frequency shift can be avoided altogether. This method is the result of a cooperation between scientists from PTB, the National Physical Laboratory (NPL) in Great Britain and the National Institute of Standards and Technology (NIST) in the USA. By a slight adjustment of the power of the microwave radiation exciting the transition in the caesium atom, the cumulative collisional shift can be tuned from negative to positive values – or be made precisely zero. The physical principle exploited here depends on the way in which the characteristics of the collisions between the cold atoms change qualitatively and quantitatively while the atomic cloud is flying through the apparatus. This physical model has been developed and tested on PTB‘s CSF1 and NPL‘s CsF1 fountain clocks and could be confirmed by NPL scientists by means of numerical simulations using the collision physics data calculated by the NIST researchers.

This opens up the fascinating prospect of operating caesium fountain clocks with an exactly compensated collisional shift so that an explicit correction is no longer necessary. Currently, further investigations are being made to determine the practical limits of such zero-collisional shift. But already now one can anticipate caesium fountain clocks with much better performance than previously thought possible.

Contact at PTB:

Phone: +49-531-592-0