Laser-cooled Cs frequency standard and a measurement of the frequency shift due to ultracold collisions

We demonstate 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 μK. The measured shift is -12.9±0.7 mHz for a density of (2.7±1.5)×${10}^9$ ${\mathrm{cm}}^{\mathrm{-}3}$ beginning with an even distribution (±10%) among the F=3, ${\mathit{m}}_{\mathit{F}}$ states. For a fountain with 95% of the atoms in the ${\mathit{m}}_{\mathit{F}}$=0 states, we measure a frequency shift of -5.5±0.5 mHz at a density of (3.5±2.0)×${10}^8$ ${\mathrm{cm}}^{\mathrm{-}3}$.