Gauge potentials and tunneling dynamics in optical lattices

Summary form only given. It has been predicted that topological potentials play a significant role in gray optical lattices. However, so far there has been no direct experimental demonstration of their existence. Unlike light-shift potentials, which, in the case of low saturation, scale as the laser intensity, topological (or gauge) potentials are intensity independent and solely depend on the topology of the lattice, which is determined by the detuning, beam geometry and magnetic fields. In our experiment, /sup 87/Rb atoms are collected in a standard magneto-optic trap and loaded into a 1D optical lattice. The lattice is formed by counterpropagating, orthogonally polarized laser beams. The minima of the trapping potential form at the antinodes of the standing waves and, as a result of optical pumping, the wells are alternately occupied. When an atom tunnels from one type of well to another, it must then flip its magnetic spin state. The gauge potential is associated with the energy needed for this change. In this gray lattice, the potential wells are shallow due to the weak interaction with the light field. Shallow potentials are necessary for tunneling. We use two schemes to observe tunneling. In the first, we use an auxiliary weak laser pulse to selectively depopulate one of the two types of wells. We observe tunneling by measuring the population in each type of well as a function of time. The second way we study tunneling is to create atomic wave-packets within the wells and to observe their motion as they oscillate. The wave-packets are created by diabatically displacing the trapped atoms in either position or momentum space. Tunneling occurs when the wave-packets run against barriers provided by the maxima of the trapping potential.