Equivalent classes of closed three-level systems - art. no. 015802

In recent years a significant amount of research in quantum optics has been devoted to the analysis of atomic three-level systems and for many physica l quantities the same effects have been predicted for different configurati ons. These configurations can be split into essentially two classes. One fo r which the system contains a metastable state and another where the system has two close-lying levels and coherence effects become important. We demo nstrate when and why for a wide range of parameters these two classes are, in fact, equivalent for many important physical quantities. A unified pictu re underlying a large body of work on these categories of atomic three-leve l systems is presented and applied to some examples.