An experimental scheme for discrete-time quantum walks is proposed for a spinor Bose-Einstein condensate to steer through the Brillouin zone, where the position of the walker is encoded in the quasimomentum.
We investigate the possibility of implementing a sequence of quantum walks whose probability distributions give an overall positive winning probability, while it is negative for the single walks (Parrondo’s paradox). In particular, we have in mind an experimental realization with a Bose–Einstein condensate in which the walker’s space is momentum space. Experimental problems in the precise implementation of the coin operations for our discrete-time quantum walks are analyzed in detail. We study time-dependent phase fluctuations of the coins as well as perturbations arising from the finite momentum width of the condensate. We confirm the visibility of Parrondo’s paradox for experimentally available time scales of up to a few hundred steps of the walk.
We study quantum effects in the dynamics of a Bose-Einstein condensate loaded onto the edge of a Brillouin zone of a one-dimensional periodic potential created by an optical lattice. We show that quantum fluctuations trigger the dynamical instability of the Bloch states of the condensate and can lead to the generation of arrays of matter-wave gap solitons. Our approach also allows us to study the instability-induced anomalous heating of the condensate at the edge of the Brillouin zone and growth of the uncondensed atomic fraction. We demonstrate that there are regimes in which the heating effects do not suppress the formation of the localised states. We show that a phase imprinting technique can ensure the formation of gap soliton trains after short evolution times and at fixed positions.