Abstract
The preparation of a sample of ultra cold caesium atoms in an ${m}_{F}=0$ Zeeman sublevel of the $F=3$ ground state is demonstrated, using cooling in an optical lattice and adiabatic passage. 97% of the atoms finally detected in the experiment populate the ${m}_{F}=0$ sublevel at a temperature of $300\phantom{\rule{0.3em}{0ex}}\mathrm{nK}$. Microwave spectroscopy is used to measure the adiabatic transfer directly and its dependence on various experimental parameters. A theoretical model is provided to estimate the upper limit for the efficiency of adiabatic transfer in a multilevel system with nonresonant excitation. The results constitute significant progress towards constructing a caesium fountain frequency standard operating in the nK temperature range.
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