We have measured the attractive van der Waals force between a dielectric wall and an atom in its ground state. The method is a direct force measurement in which we use an evanescent wave atomic mirror to balance the van der Waals force and the inertia of the incident atom. [S0031-9007(96) PACS numbers: 42.50.Vk, For many years, the van der Waals interaction between a ground state atom and a wall-dielectric or conductor-has attracted a lot of theoretical attention. Even the simple Lennard-Jones model [1] based on the electrostatic interaction between the atomic dipole and its image involves the quantum fluctuations of the atomic dipole. It was recognized by Casimir and Polder [2] that when the atom-wall distance z is not small compared to the wavelengths of the dominant atomic transitions, the z 23 law associated with the instantaneous electrostatic interaction is no longer valid. The full quantum treatment of the van der Waals attraction, leading to the famous long distance z 24 law, is a fundamental QED problem [3] involving the quantized electromagnetic field and retardation effects. The van der Waals energy shift can be considered a modification of the Lamb shift resulting from the modification of the density of modes of the electromagnetic field due to the presence of the wall. In the case of a dielectric wall, this density must take into account not only modes associated with traveling waves incident on, and reflected from, the vacuum-dielectric interface, but also evanescent waves [4,5].In contrast to the theoretical work, few experimental results have been reported on the van der Waals interaction between an atom and a wall. The pioneering experiments of Ref.[6] studied the deflection of thermal atomic beam by a sharp metal or dielectric edge. The observed effect was extremely weak, because only a very small fraction of the atoms passed close enough to the interface to have an interaction energy comparable to their kinetic energy. Qualitative trends in agreement with the z 23 law were observed, but no precise quantitative comparison was possible. Recently, more precise data on the van der Waals interaction between an atom and a metal has been obtained by spectroscopic studies of Rydberg atoms in a micron-sized parallel-plate metallic cavity [7]. A study of the transmission of ground state atoms through a similar cavity also permitted the measurement of the Casimir Polder force on an atom in its ground state [8]. Another series of spectroscopic measurements on light reflected from the wall of a cell containing an atomic vapor has given information on the difference between the van der Waals shifts of various atomic levels, and interesting re-sults have been obtained on the role of the frequency dependence of the dielectric constant of the wall [9].In this paper, we report on new mechanical measurements for determining the van der Waals interaction between a ground state atom and a dielectric wall. The idea, first used in Ref.[10], is to release laser cooled atoms with a well-defined kinetic energy onto an ...

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