Adiabatic passage in an open multilevel system

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 $300\mathrm{nK}$. 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.