Tunneling Dynamics and Gauge Potentials in Optical Lattices

We study periodic well-to-well tunneling of ${}^{87}\mathrm{Rb}$ atoms on adiabatic potential surfaces of a 1D optical lattice. The observed dependence of the lowest-band tunneling period on the depth of the adiabatic potential can only be explained by an additional intensity-independent gauge potential predicted by Dum et al. The experimental data are in excellent agreement with our quantum Monte Carlo wave-function simulations and band structure calculations.