Pure superposition states of atoms generated by a bichromatic elliptically polarized field

We identify the specific polarizations for the two components of a bichromatic field, which produce pure superposition states of atoms with a specific magnetic quantum number m via coherent population trapping. The superposition states are composed of two Zeeman substates magnetic quantum number m of the two ground-state hyperfine levels with arbitrary angular momenta F{sub 1} and F{sub 2}. It is established that in the general case of m{ne}0, optical fields with elliptical polarizations are needed for the preparation of such a pure state. It is shown analitically that a unique advantage of the D1 line (over the D2 line) of alkali-metal atoms is the possibility that pure m-m states may be generated even if the excited-state hyperfine levels are not spectrally resolved.