Decoherence of Josephson-Type oscillations in a Bose-Einstein condensate driven through an optical lattice

Experiments on Bose-Einstein condensates moving through optical lattices have allowed the observation of a number of phenomena associated with coherent atomic transport via matter waves. In an earlier work [M.L. Chiofalo, M. Polini, and M.P. Tosi, Laser Phys. 12, 50 (2002)], the Gross-Pitaevskii equation was used to treat an atomic condensate driven by a harmonic force to move through a periodic potential representing a quasi-one-dimensional optical lattice. It was shown that, for moderate values of the strength of the driving force and of the potential barrier height, the alternating matter current can be mapped into the superfluid current passing through a Josephson junction in an ac field. Here, we report on a numerical study aimed at showing how such coherent transport behavior breaks down as (i) the strength of the harmonic force is increased or (ii) the barrier height of the lattice potential is raised. In either case, phase coherence between the atomic clouds inside adjacent potential wells is lost. The emergence of decoherence is examined in both coordinate and momentum space.