Experimental observation of subabsorption
We predict and experimentally demonstrate a new 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 $^{87}$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 $\mu$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 density-dependent dephasing rate of the long-range dipole-dipole correlations as a single adjustable parameter. Experiment-theory comparison indicates a dephasing coefficient of $\beta/2 \pi = 4.9 \times 10^{-5}$ Hz~cm$^3$, which is more than two orders of magnitude larger than the known dipole-dipole line broadening coefficient in $^{87}$Rb.