Light shift and Fano resonances in a single cold ion

The simultaneous excitation of adjacent atomic transitions by two light waves equally detuned from their resonances gives rise to destructive interference of the transitions and causes a narrow dark line, or coherent dip, to appear in both light scattering and absorption. With one of the light fields detuned from resonance, the line is asymmetric and is accompanied by a shifted bright line; the two are represented by a Fano line profile. The spectral separation ∊ of the bright and dark lines is closely related—although not identical—to the net shift that is found in the two-photon-resonant denominator of the susceptibility, to the shift of the normal modes of the atom’s two internal excitations, and to the net difference of the energy separations of dressed and bare atomic levels. We report measurement of the resonance shift ∊ of the 2S1/2–2D3/2 transition of a single cooled and trapped Ba+ ion, and we analyze its relationship to those identified light shifts.