State selection in atomic caesium

Primary frequency standards operate with atoms in states which are insensitive to magnetic fields (m_F = 0 states). There are several techniques that can be used to prepare an atomic sample in the proper internal state.

In caesium frequency standards, selection of atoms in the m_F = 0 state is normally achieved by removing the atoms that lie in the other m_F states. After launch, the atomic population is approximately evenly distributed among all the magnetic sublevels of the F = 4 state, with about 12% of the population in the m_F = 0 state. These atoms are transferred, with 95% efficiency, to the (F = 3, mF = 0) state by a 5 ms pulse of a travelling microwave beam. Atoms remaining in the F = 4 state are pushed by the radiation pressure from a 0.5 ms pulse of a vertical beam, tuned to the resonance. The selected (F = 3, m_F = 0) atoms continue their flight into the magnetically shielded flight tube.

Adiabatic passage offers an attractive alternative solution for the efficient transfer of atomic population between Zeeman sublevels of the ground state. The transfer is realised by subjecting atoms to two delayed optical fields. For given initial and final states of the ensemble of atoms, the parameters of the light fields (detuning, polarisation) are chosen so that the atoms are kept in a non-absorbing (dark) state throughout the process. The transfer of the atomic population to the m_F = 0 state preserves the temperature of the atoms.

We have demonstrated adiabatic passage between the m_F = 3 and m_F = 0 states. First a σ⁺ polarised pulse, tuned to the F = 3 → F’ = 3 resonance, is applied. For this pulse the initial m_F = 3 state is a dark state. As the intensity of that pulse decreases, an orthogonal π-polarised pulse is applied, for which the final m_F = 0 state is a dark state. The intensities of the two pulses, as a function of time, have approximately Gaussian shape (half-width 5 μs) and the peaks are offset by 5 ms. The RF spectrum after the adiabatic passage indicates that 85% of the F = 3 atoms are in the m_F = 0 sublevel. 40 % of the initial number of atoms are lost from F = 3 due to optical pumping to F = 4 by off-resonant excitation of the F’ = 4 state. These atoms are easily removed from the fountain by radiation pressure; however off-resonant excitation could be reduced by applying the adiabatic passage using pulses tuned to the F = 3 → F’ = 3 transition on the caesium D1 line, where the hyperfine splitting is larger.

For an atomic sample at a temperature of 1-2 μK, simple optical pumping could be implemented to transfer the atom to the m_F = 0 state. The optimal strategy for state preparation of the atomic sample in an atomic fountain frequency standard is currently being defined.