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Atomic fountain clocks are becoming still more accurate

PTB scientists have found out how to cancel the collisinal frequency shift in atomic fountain clocks which constitutes a major part of the total inaccuracy

21.06.2007

It was thought to date that the accuracy of the currently best atomic clocks, the caesium fountains, could not be significantly increased any more. That is no longer true. With a series of experiments on its caesium fountain clck CSF1, Scientists of the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig have found a previously unknown effect connected to the mutual collisions of the caesium atoms in the clock. The results, obtained in collaboration with British and US-American colleagues and published in the journal Physical Review Letters, imply a substantial performance increase for caesium fountain clocks

Abbildung: Die beiden Caesium-Fontänenuhren in der Uhrenhalle der PTB Braunschweig gehören zu den derzeit genauesten Uhren der Welt. (Das Foto findet sich hier 72-dpi-Datei oder kann als 300-dpi-Datei bei der Presse- und Öffentlichkeitsarbeit bestellt werden: erika.schow@ptb.de)

In the international system of units (SI) the second is defined via a specific microwave transition between two internal energy levels in the caesium atom. In a caesium fountain clock, the best primary atomic clocks available today, laser beams help to collect caesium atoms in a small cloud, to cool them, and to throw them vertically up by about a metre before they fall back. During this free-flight period the frequency of the transition can be determined with great accuracy: fountain clocks today reproduce the length of the SI second to within better than 15 significant digits.

A key point for the operation of any primary atomic clock is that all effects that might disturb the resonance frequency of the atoms need to be understood in detail, so that the resulting frequency shifts can be avoided or corrected for. A particularly important correction for a caesium fountain clock becomes necessary because of the mutual collisions of the cold atoms in the cloud. In general, the inaccuracy of this correction constitutes a major part of the total inaccuracy of the fountain clock.

Now a method has been devised to eliminate the collisional frequency shift altogether. This result, obtained in a collaboration between scientists from PTB, the National Physical Laboratory (NPL) in Great Britain, and the National Institute of Standards and Technology (NIST) in USA, has recently been published in the journal Physical Review Letters.

The new method was developed using the fountain clocks PTB-CSF1 and NPL-CsF1. By slightly adjusting the power of the microwave radiation exciting the clock transition in the caesium atoms one can change the value of the total collisional frequency shift from negative to positive. or tune it to zero precisely! The physical mechanism behind this effect is connected to the way in which the collisions among cold atoms change qualitatively and quantitatively during the cloud. s flight through the apparatus. This could be confirmed by Monte-Carlo simulations performed by the NPL scientists, using atomic-collision data calculated by the NIST scientists.

As a consequence, there is now the intriguing prospect of operating the caesium fountain clock such that the collisional frequency shift is exactly compensated, so that a correction for this effect is no longer necessary. Currently investigations are on-going to find out what the practical limitations of operation at the zero-shift point are. But already now one can anticipate caesium clocks with much better performance than previously thought possible.


Cancellation of the Collisional Frequency Shift in Caesium Fountain Clocks.
K. Szymaniec, W. Chalupczak, E. Tiesinga, C.J. Williams, S. Weyers, and R. Wynands
Physical Review Letters 98, 153002 (2007)

Contact:
Dr. Robert Wynands,
PTB Working Group Unit of Time,
Tel. (0531) 592-4414
E-Mail: time(at)ptb.de