The goal of this experiment is to spectroscopically measure energy level shifts that result from Rydberg atom interactions. As described in the introduction, there are two, fundamentally different interaction regimes. The van der Waals regime occurs when the interactions between the atoms are much less in magnitude then the energy detuning D. The dipole-dipole regime occurs in the opposite case, when the interactions are much stronger than the energy detuning.
The level shifts for the two regimes have a qualitatively different structure. To begin, the shifted energy levels are bands of states instead of discrete states because the contributions to the shifts come from atoms separated by many different distances. Additionally, in the dipole-dipole case, we expect to see two symmetrically split states, equally displaced from the interaction free energy shown by the dotted line in the figure to the left. However, in the van der Waals case, we expect to only see one of these shifted states. For more information, click here.