Trapping Atoms by the Vacuum Field in a Cavity

We analyse the force on an atom resonantly coupled to the vacuum field in a high-Q microwave cavity. The atom is prepared in the upper level of the atomic transition coupled to the cavity mode. The force results from the reversible exchange of a single excitation between the atom and the cavity. It derives from a potential which reflects the spatial variations of the r.m.s. vacuum field inside the cavity. Depending upon the atom-cavity dressed state into which the system is prepared, this potential presents either a maximum or a minimum at the point where vacuum fluctuations are largest. Very slow circular Rydberg atoms may be reflected back as they enter the cavity, or trapped around cavity centre.