  In this article we theoretically study the phase shift a single atom imprints
onto a coherent state light beam in free space. The calculations are performed
in a semiclassical framework. The key parameters governing the interaction and
thus the measurable phase shift are the solid angle from which the light is
focused onto the atom and the overlap of the incident radiation with the atomic
dipole radiation pattern. The analysis includes saturation effects and
discusses the associated Kerr-type non-linearity of a single atom.
