  Optical double-quantum two-dimensional coherent spectroscopy (2DCS) was
implemented to probe interatomic dipole-dipole interactions in both potassium
and rubidium atomic vapors. The dipole-dipole interaction was detected at
densities of $4.81 \times 10^8$ cm$^{-3}$ and $8.40 \times 10^9$ cm$^{-3}$ for
potassium and rubidium, respectively, corresponding to a mean interatomic
separation of 15.8 $\mu$m or $3.0\times 10^5a_0$ for potassium and 6.1 $\mu$m
or $1.2\times 10^5a_0$ for rubidium, where $a_0$ is the Bohr radius. We report
the lowest atomic density at which dipole-dipole interactions are detected. The
experimental results confirm the long range nature of the dipole-dipole
interaction which is critical for understanding many-body physics in
atoms/molecules. The long range interaction also has implications in atom-based
applications involving many-body interactions. Additionally, we demonstrated
that double-quantum 2DCS is sufficiently sensitive to probe dipole-dipole
interaction at densities that can be achieved with cold atom in a
magneto-optical trap, paving the way for double-quantum 2DCS studies of cold
atoms and molecules. The method can also open a new avenue to study long-range
interactions in solid states systems such as quantum dots and color centers in
diamonds.
