LASER-COOLED CS FREQUENCY STANDARD AND A MEASUREMENT OF THE FREQUENCY-SHIFT DUE TO ULTRACOLD COLLISIONS

We demonstrate a laser-cooled Cs fountain which can be used as a frequency standard. The fountain has been used to measure the frequency shift due to ultracold Cs-Cs collisions at temperatures below 2.8 muK. The measured shift is -12.9 +/- 0.7 mHz for a density of (2.7 +/- 1.5) x 10(9) cm-3 beginning with an even distribution (+/- 10%) among the F = 3, m(F) states. For a fountain with 95% of the atoms in the m(F) = 0 states, we measure a frequency shift of -5.5 +/- 0.5 mHz at a density of (3.5 +/- 2.0) x 10(8) cm-3.