Approaching classicality in quantum accelerator modes through decoherence - art. no. 056233

We describe measurements of the mean energy of an ensemble of laser-cooled atoms in an atom optical system in which the cold atoms, falling freely und er gravity, receive approximate delta -kicks from a pulsed standing wave of laser light. We call this system a "delta -kicked accelerator." Additional ly, we can counteract the effect of gravity by appropriate shifting of the position of the standing wave, which restores the dynamics of the standard delta -kicked rotor. The presence of gravity (delta -kicked accelerator) yi elds quantum phenomena, quantum accelerator modes, which are markedly diffe rent from those in the case for which gravity is absent (delta -kicked roto r). Quantum accelerator modes result in a much higher rate of increase in t he mean energy of the system than is found in its classical analog. When gr avity is counteracted, the system exhibits the suppression of the momentum diffusion characteristic of dynamical localization. The effect of noise is examined and a comparison is made with simulations of both quantum-mechanic al and classical versions of the system. We find that the introduction of n oise results in the restoration of several signatures of classical behavior , although significant quantum features remain.