Vahid Sandoghdar

The attraction of an atom to metallic surfaces has been common knowledge for more than 150 years. In the framework of quantum electrodynamics, and to a first approximation, this interaction is caused by the coupling of the atomic dipole moment with the modified electromagnetic field in the presence of the boundary. When the atom-boundary separations are much smaller than the Bohr wavelengths of the atomic state, this interaction is non-retarded and well characterized by the electrostatic model of the Lennard-Jones theory. In this work we have studied such an interaction between a sodium atom and the walls of a gold-coated parallel-plate cavity. An atomic beam of ground state sodium is sent through a cavity where the atoms are excited to a low-lying Rydberg state in a two step resonant transition process. The Rydberg atoms are then counted, and the signal is plotted as a function of second step laser frequency. By comparing the excitation spectra obtained from inside the cavity with those recorded in an unperturbed atomic beam, we are able directly to measure the energy level shift of these Rydberg states. We then vary the cavity width from about one half to more than two micrometers and perform the experiment for four principal quantum numbers (n = 10-13). The data for the smallest cavity width (504nm) was obtained from atoms channeled at the center of the cavity by means of an optical dipole force produced by a yellow laser beam in a standing wave configuration across the cavity walls. This compression of atomic beam is instrumental for the revival of the signal which becomes drowned in noise for small cavities. In conclusion, we verify every aspect of the Lennard-Jones model, including its dependence on inverse cube of distance and its proportionality to the square of the atomic dipole moment. This provides the first comprehensive test of the Lennard-Jones van der Waals theory.