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arXiv:physics/0209010 · Physical Review Letters

Controlling the Cold Collision Shift in High Precision Atomic Interferometry

Franck Pereira dos Santos, H. Marion, S. Bize, Yvan R. P. Sortais, Andr  Clairon, Ch Salomon

Published Nov 18, 2002• 110 citations
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Abstract

We present a new method based on a transfer of population by adiabatic passage that allows one to prepare cold atomic samples with a well-defined ratio of atomic density and atom number. This method is used to perform a measurement of the cold collision frequency shift in a laser cooled cesium clock at the percent level, which makes the evaluation of the cesium fountain accuracy at the 10(-16) level realistic. With improvements, the adiabatic passage would allow measurements of density-dependent phase shifts at the 10(-3) level in high precision experiments.

DOI
10.1103/physrevlett.89.233004

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We present a new method based on a transfer of population by adiabatic passage that allows one to prepare cold atomic samples with a well-defined ratio of atomic density and atom number. This method is used to perform a measurement of the cold collision frequency shift in a laser cooled cesium clock at the percent level, which makes the evaluation of the cesium fountain accuracy at the 10(-16) level realistic. With improvements, the adiabatic passage would allow measurements of density-dependent phase shifts at the 10(-3) level in high precision experiments.
For a long time, one of the main limitations of cesium atomic fountains has been the cold collision frequency shift. By using a method based on a transfer of population by adiabatic passage (AP) allowing to prepare cold atomic samples with a well defined ratio of atomic density as well as atom number we have a better measurement of this effect. By improving the method we found out some unexpected properties of cold collisions, confirming that 133Cs is a rich atom to study cold collisions. Those results lead to fine comparisons with cold collision theory and may constraint some parameters of cesium.
A technique to cancel the frequency shift due to cold collisions of laser-cooled atoms is proposed. Results for future clocks based on 135Cs^{135}\mathrm{Cs}135Cs and 137Cs^{137}\mathrm{Cs}137Cs are presented. We find a 137Cs^{137}\mathrm{Cs}137Cs fountain clock can be operated with no cold-collision shift by adjusting the population ratios in the fountain, and it may also be possible for 135Cs^{135}\mathrm{Cs}135Cs. This technique can also be applied to other phase-sensitive measurements using cold atoms, including atom interferometers.
The spin-exchange frequency shift due to cold collisions between ground-state atoms appears to be the systematic effect which could limit the accuracy of a laser-cooled cesium fountain operating as a frequency standard. We have investigated this shift for atoms distributed among all the Zeeman substates of either the F = 4 or F = 3 hyperfine level of the 6 S 1/2 ground state of cesium and for the case where most of the atoms are prepared in F = 3, M F = 0 sublevel. The measured relative frequency shifts are respectively − 1.0·10 −21 , − 0.6·10 −21 and − 2.0·10 −21 (at/cm 3 ) −1 . These results agree with the measurements performed by Gibble and Chu at higher atom densities with atoms prepared in F = 3, M F = 0 sublevel. However, the precision of our frequency measurements is much better and is only limited at 2·10 −15 by the H-maser used as a reference oscillator. Finally, we show that the relative frequency shift due to cold collisions can be reduced to 10 −15 , while preserving a short-term frequency stability of about 10 −13 τ −1/2 .
One of the main limitations of cesium atomic fountains has been the cold collision frequency shift. By using a method based on a transfer of population by adiabatic passage allowing to prepare cold atomic samples with a well defined ratio of atomic density as well as atom number the collisional shift is controlled at the 10E-3 of its value. A calibration of Zeeman sub-states contribution to the clock shift as a function of the field has been performed. Feshbach resonances have been observed for the first time at very low magnetic field and with a very good resolution. A Monte Carlo simulation has been performed and could fit properly some of experimental data. This constrains some parameters of the theory of collisions.