Collective atomic recoil in a moving Bose-Einstein condensate: From superradiance to Bragg scattering

We present the results of an experiment on light scattering from an elongated Bose-Einstein condensate (BEC) interacting with a far-off-resonant pump laser. By collective atomic recoil lasing (CARL) a coherent superposition of two atomic wave packets with different momenta is created. Varying the intensity of a weak counterpropagating laser beam we observe the transition from the pure superradiant regime to the Bragg scattering regime, where Rabi oscillations in a two-level system are observed. The process is limited by the decoherence between the two atomic wave packets. In the superradiant regime the experiment gives evidence of a contribution to decoherence which depends on the initial velocity of the condensate. The system is described by the CARL-BEC model, which is a generalization of the Gross-Pitaevskii model to include the self-consistent evolution of the scattered field and a phase-diffusion decoherence process, which accounts for the observed damping.