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

Abstract

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 \ensuremath{\mu}K. The measured shift is -12.9\ifmmode\pm\else\textpm\fi{}0.7 mHz for a density of (2.7\ifmmode\pm\else\textpm\fi{}1.5)\ifmmode\times\else\texttimes\fi{}${10}^{9}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ beginning with an even distribution (\ifmmode\pm\else\textpm\fi{}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\ifmmode\pm\else\textpm\fi{}0.5 mHz at a density of (3.5\ifmmode\pm\else\textpm\fi{}2.0)\ifmmode\times\else\texttimes\fi{}${10}^{8}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$.