  One of the most striking features of the strong interactions between Rydberg
atoms is the dipole blockade effect, which allows only a single excitation to
the Rydberg state within the volume of the blockade sphere. Here we present a
method that spatially visualizes this phenomenon in an inhomogeneous gas of
ultra-cold rubidium atoms. In our experiment we scan the position of one of the
excitation lasers across the cold cloud and determine the number of Rydberg
excitations detected as a function of position. Comparing this distribution to
the one obtained for the number of ions created by a two-photon ionization
process via the intermediate 5P level, we demonstrate that the blockade effect
modifies the width of the Rydberg excitation profile. Furthermore, we study the
dynamics of the Rydberg excitation and find that the timescale for the
excitation depends on the atomic density at the beam position.
