Pure superposition states of atoms generated by a bichromatic   elliptically polarized filed
  We find specific polarizations of components of a bichromatic field, which allow one to prepare pure superposition states of atoms, using the coherent population trapping effect. These $m$$-$$m$ states are prepared in the system of Zeeman substates of the ground-state hyperfine levels with arbitrary angular momenta $F_1$ and $F_2$. It is established that, in general case $m\ne 0$, the use of waves with elliptical polarizations ($\epsilon_1$$\perp$$\epsilon_2$ field configuration for alkali metal atoms) is necessary for the pure state preparation. We analytically show an unique advantage of the D1 line of alkali metal atoms, which consists in the possibility to generate pure $m$$-$$m$ states even in the absence of spectral resolution of the excited-state hyperfine levels, contrary to the D2 line. 
A.V. Taichenachev, V.I. Yudin, V.L. Velichansky, A.S. Zibrov