Krzystof Szymaniec Abstract

Krzysztof Szymaniec
National Physical Laboratory, UK

"Cancellation of the Collision Frequency Shift in Caesium Fountain Clocks"

The invention of laser cooling enabled realization of fountain-type microwave frequency standards (clocks) and more than ten-fold accuracy improvement over the best thermal beam devices. On the other hand, collisions between the atoms, usually negligible in the beam standards, have become a major issue because for the laser-cooled atoms the de Broglie wavelength is much larger than the range of inter-atomic potentials. Usually a correction for this shift is obtained by measuring the frequency of the standard as a function of atomic density and performing an extrapolation to zero density. In general this means that additional measurement time is needed to reach a given statistical resolution. Recently, we have observed that, for a certain range of fountain parameters (i.e. the initial size of the atom cloud and its temperature at launch), the collisional frequency shift varies significantly when the relative population of the clock states is varied. In particular, the collisional shift can be zero for a certain value of the population ratio. This demonstration of collisional shift cancellation offers the intriguing prospect of operating the fountain at the zero-shift point, avoiding the need for extrapolation. Depending on experimental details, a shortening of the averaging time by an order of magnitude is possible.

I will describe the purpose and operation of the Cs fountain primary clock., present the mechanism for the collisional shift cancellation and discuss the expected improvement of the fountain's performance.