Beam quality of a non-ideal atom laser
  We study the propagation of a non-interacting atom laser distorted by the strong lensing effect of the Bose-Einstein Condensate (BEC) from which it is outcoupled. We observe a transverse structure containing caustics that vary with the density within the residing BEC. Using WKB approximation, Fresnel-Kirchhoff integral formalism and ABCD matrices, we are able to describe analytically the atom laser propagation. This allows us to characterize the quality of the non-ideal atom laser beam by a generalized M$^2$ factor defined in analogy to photon lasers. Finally we measure this quality factor for different lensing effects. 
J.-F. Riou (CHARLES Fabry), W. Guerin (CHARLES Fabry), Y. Le Coq   (CHARLES Fabry), M. Fauquembergue (CHARLES Fabry), Philip...