Decoherence effects in superradiant light scattering from a moving Bose-Einstein condensate

We present the results of an experiment on superradiant Rayleigh scattering from a moving Bose-Einstein condensate, where a superposition of two atomic wavepackets is created by the interaction of a far-detuned laser beam with 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. The experiment gives evidence of a damping of the matter-wave grating which depends on the initial velocity of the condensate. We describe this damping in terms of a phase-diffusion decoherence process, in good agreement with the experimental results. In a second experiment we add a seeding beam counterpropagating with respect to the pump beam. Varying intensity and frequency of the seed, we investigate the transition from the superradiant regime to the Rabi oscillations regime, evidencing fundamental differences between these two processes.