High-order nonlinearities in the motion of a trapped atom

We study the counterpart to the multiphoton down-conversion in the quantize d motion of a trapped atom. The Lamb-Dicke approximation leads to a diverge nce of the mean motional excitation in a finite interaction time for k-quan tum down-conversions;with k greater than or equal to 3, analogous to the si tuation in the parametric,approximation of nonlinear optics. We show that, in contrast to the Lamb-Dicke approximation, the correct treatment of the o verlap of the atomic center-of-mass wave function and the driving laser wav es leads to a proper dynamics without any divergence problem. That is, the wavy nature of both matter and light is an important physical property that cannot be neglected for describing the motional dynamics of a trapped atom , even for small Lamb-Dicke parameters. [S1050-2947(99)00301-7].