  The long-range dipole-dipole interaction can create delocalized states due to
the exchange of excitation between Rydberg atoms. We show that even in a random
gas many of the single-exciton eigenstates are surprisingly delocalized,
composed of roughly one quarter of the participating atoms. We identify two
different types of eigenstates: one which stems from strongly-interacting
clusters, resulting in localized states, and one which extends over large
delocalized networks of atoms. These two types of states can be excited and
distinguished by appropriately tuned microwave pulses, and their relative
contributions can be modified by the Rydberg blockade and the choice of
microwave parameters.
