  In the laser excitation of ultracold atoms to Rydberg states, we observe a
dramatic suppression caused by van der Waals interactions. This behavior is
interpreted as a local excitation blockade: Rydberg atoms strongly inhibit
excitation of their neighbors. We measure suppression, relative to isolated
atom excitation, by up to a factor of 6.4. The dependence of this suppression
on both laser irradiance and atomic density are in good agreement with a
mean-field model. These results are an important step towards using ultracold
Rydberg atoms in quantum information processing.
