Single atom in free space as a quantum aperture

We calculate exact three-dimensional solutions of Maxwell equations corresponding to strongly focused light beams, and study their interaction with a single atom in free space. We show how the naive picture of the atom as an absorber with a size given by its radiative cross section $\sigma =3\mathrm{}{\lambda }^2/2\mathrm{}\pi$ must be modified. The implications of these results for quantum-information-processing capabilities of trapped atoms are discussed.