We predict and experimentally demonstrate a type of collective (cooperative) coupling effect where a disordered atomic ensemble absorbs light with a rise time longer (i.e., at a rate slower) than what is dictated by single-atom physics. This effect, which we name subabsorption, can be viewed as the absorptive analog of subradiance. The experiment is performed using a dilute ensemble of ultracold ${}^{87}\mathrm{Rb}$ atoms with a low optical depth, and time resolving the absorption of a weak (tens of photons per pulse) resonant laser beam. In this dilute regime, the collective interaction relies on establishing dipole-dipole correlations over many atoms; i.e., the interaction is not dominated by the nearest neighbors. As a result, subabsorption is highly susceptible to motional dephasing: even a temperature increase of $60\mu \mathrm{K}$ is enough to completely extinguish the subabsorption signal. We also present a theoretical model whose results are in reasonable agreement with the experimental observations. The model uses the density-dependent dephasing rate of the long-range dipole-dipole correlations as a single adjustable parameter. Experiment-theory comparison indicates a dephasing coefficient of $\beta /2\pi =4.9\times {10}^{-5}\mathrm{Hz}{\mathrm{cm}}^3$, which is more than two orders of magnitude larger than the known dipole-dipole line broadening coefficient in ${}^{87}\mathrm{Rb}$.

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• Effects of atomic coherence on light propagation
• Superradiance & subradiance