Temperature dependence of the atom-surface interaction in thermal equilibrium

Summary form only given. Experimental studies of the atom-surface or surface-surface interactions provide a physical insight on the interaction of quantum bodies with the fluctuating vacuum. They can be of great practical interest in the ever expanding field of nanotechnologies. In particular the dependence of the Casimir and Casimir-Polder interaction on the thermal fluctuations is a fundamental effect that could in the future provide a useful knob for tuning these interactions. Temperature effects are usually very small and have so far been observed on a BEC placed 6-10 μm away from a fused-silica substrate due to an out of thermal equilibrium enhancement. Here we report on spectroscopic selective reflection measurements of the interaction between a Cs (7D3/2) atom and a sapphire surface. A virtual coupling between a surface resonance of sapphire at 12.1 μm and the Cs 7D3/2Æ5F5/2 transition at 10.8 μm almost eliminates the resonant contribution for this dipole transition. Reaching the temperatures required to observe an effect is extremely challenging. For this reason our experiment is performed in an all-sapphire vapour cell with super polished windows with an annealing post-treatment. The interaction is probed up to unusually high temperatures reaching more than 1000 K. Experimental measurements show C₃ coefficient as a function of temperature compared to a full QED calculation. Each point is deduced by fitting reflection spectra to a theoretical model and is averaged for different vapour densities and different spots on the window. Statistical and systematic uncertainties have been reduced compared to our previous experiments performed on a different all-sapphire cell with windows of uncertain surface quality. The chemical stability of sapphire under these extreme conditions seems to be a key factor for this experimental achievement. Previous measurements between Cs (8P_3/2) and a CaF₂ surface in - specially designed cell were unsuccessful due to chemical deterioration of the surface. This experiment is the first to provide a sensitive measurement of temperature effects on the atom-surface interaction in thermal equilibrium and highlights the QED nature of van der Waals regime.