J. Keaveney et al. [1]

Figure 1: A focused laser beam (red) probes an ensemble of Rubidium atoms (yellow) confined between two windows of a vapor cell. The average distance of the atoms is smaller than the range of their dipole-dipole interaction (green sphere). Under these conditions the spectrally resolved transmission of the laser radiation reveals the collective Lamb shift of the atoms, shown in the bottom graph as function of the size of the atomic ensemble, defined by the distance $D$ of the windows (in units of the radiation wavelength).