Tunnelling and the Born-Oppenheimer approximation in optical lattices

We study periodic well-to-well flopping of rubidium atoms in one-dimensiona l grey optical lattices using a nondestructive, real-time measurement techn ique and quantum Monte Carlo wavefunction simulations. The observed floppin g rates as well as flopping rates extracted from exact band structure calcu lations can largely be reproduced using adiabatic models that employ the Bo rn-Oppenheimer approximation. The adiabatic model is greatly improved by ta king into account a gauge potential that is added to the usual adiabatic li ght-shift potential. The validity of the adiabatic model allows us to inter pret the observed flopping phenomenon as periodic well-to-well tunnelling. At low intensities and in related far-off-resonant optical lattices the adi abatic model fails. There, a weak-coupling model becomes valid, which descr ibes the well-to-well flopping as a Rabi oscillation between weakly coupled stales, but not as a tunnel effect.