Exciton states in a frozen Rydberg gas

The long-range dipole-dipole interaction between excited Rydberg
states of atoms can create highly delocalized states due to the exchange
of excitation between the 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.
The presence of these delocalized eigenstates could be relevant to
puzzling results in several current experiments.