Spontaneously generated coherence and dark states

It is shown that the response of a three-level atom interacting with an optical field in a "dark state geometry" (single field, degenerate ground states) depends critically on whether or not spontaneously generated coherence (SGC) is present. In the absence of SGC, the system evolves to a dark state. With SGC present, the dark state population is a constant of the motion and the steady-state values for density-matrix elements depend on their initial values at the time the field is applied.