  We report measurements of an intensity-field correlation function of the
resonance fluorescence of a single trapped Ba+ ion. Detection of a photon
prepares the atom in its ground state and we observe its subsequent evolution
under interaction with a laser field of well defined phase. We record the
regression of the resonance fluorescence source field. This provides a direct
measurement of the field of the radiating dipole of a single atom and exhibits
its strong non-classical behavior. In the experimental setup an interference
measurement is conditioned on a fluorescence photon detection. The third-order
correlation function thus recorded demonstrates an aspect of wave-particle
duality at the single-atom, single-photon level.
