Multilevel Dark States: Coherent Population Trapping with Elliptically Polarized Incoherent Light

We present the first experimental proof for the existence of elliptical dark states, the multilevel analog of the well known three-level dark states. An ensemble of multilevel atoms, when prepared by elliptically polarized light, becomes transparent to probe light of the same polarization. The effect stems from laser-induced coherences between many ground-state magnetic sublevels which reflect the polarization of the light that had created the dark state. The novelty and essential character of the dark states are elucidated by the experimental demonstration of their creation by incoherent light. It is anticipated that elliptical dark states will play an important role in the laser cooling and manipulation of molecules.