Experimental preparation of pure superposition states of atoms via   elliptically polarized bichromatic radiation
  We propose a simple and effective way of creating pure dark superposition states. The generation of pure states is carried out by using bichromatic radiation with controllable polarization ellipticity. We derived analytic formulas for polarization elipticity to obtain pure dark states of different Zeeman sublevels of alkali atoms. Experimentally we accumulated ~60% of the atoms in the 0-0 dark state of the D1 line of Rb87 
Sergei A. Zibrov, V.L. Velichansky, A.V. Taichenachev, V.I. Yudin,   A.S. Zibrov