Intensity-Field Correlation of Single-Atom Resonance Fluorescence

We report measurements of an intensity-field correlation function of the resonance fluorescence of a single trapped Ba-138(+) 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 nonclassical behavior. In the setup, an interference measurement is conditioned on the detection of a fluorescence photon.