Breakdown of the few-level approximation in collective systems

Journal article

The validity of the few-level approximation in dipole-dipole interacting collective systems is discussed. As an example system, we study the archetype case of two dipole-dipole interacting atoms, each modeled by two complete sets of angular momentum multiplets. We establish the breakdown of the few-level approximation by first proving the intuitive result that the dipole-dipole induced energy shifts between collective two-atom states depend on the length of the vector connecting the atoms, but not on its orientation, if complete and degenerate multiplets are considered. A careful analysis of our findings reveals that the simplification of the atomic level scheme by artificially omitting Zeeman sublevels in a few-level approximation generally leads to incorrect predictions. We find that this breakdown can be traced back to the dipole-dipole coupling of transitions with orthogonal dipole moments. Our interpretation enables us to identify special geometries in which partial few-level approximations to two- or three-level systems are valid. © 2007 The American Physical Society.

Authors: Kiffner, M; Evers, J; Keitel, C

• Journal: Physical Review A - Atomic, Molecular, and Optical Physics
• Volume: 76
• Issue: 1
• Publication date: 2007-07-10
• DOI: 10.1103/PhysRevA.76.013807
• EISSN: 1094-1622
• ISSN: 1050-2947
• Language: English