  We present a method to control the shape and character of the interaction
potential between cold atomic gases by weakly dressing the atomic ground state
with a Rydberg level. For increasing particle densities, a crossover takes
place from a two-particle interaction into a collective many-body interaction,
where the dipole-dipole/van der Waals Blockade phenomenon between the Rydberg
levels plays a dominant role. We study the influence of these collective
interaction potential on a Bose-Einstein condensate, and present the optimal
parameters for its experimental detection.
