Diagrammatic approach to coherent backscattering of laser light by cold   atoms: Double scattering revisited
  We present a diagrammatic derivation of the coherent backscattering spectrum from two two-level atoms using the pump-probe approach, wherein the multiple scattering signal is deduced from single-atom responses, and provide a physical interpretation of the single-atom building blocks. 
Vyacheslav Shatokhin, Thomas Wellens, and Andreas Buchleitner
physics.atom-ph quant-ph