Anisotropic fluorescence signals retarded dipole-dipole interactions in a thermal atomic cloud

We experimentally observe and theoretically explain anisotropic multiple quantum coherence signals in the fluorescence from dilute thermal potassium vapors, at room temperature and particle densities $\sim {10}^8{\mathrm{c}\mathrm{m}}^{-3}$. We identify the retarded part of the geometrically fully resolved inter-atomic, resonant dipole-dipole interaction as the crucial ingredient to theoretically reproduce all qualitative features of the experimental spectra.