  In a laser-driven Rydberg gas the strong interaction between atoms excited to
Rydberg states results in the formation of collective excitations. Atoms within
a so-called blockade volume share a single Rydberg excitation, which is
dynamically created and annihilated. For sufficiently long times this driven
system approaches a steady state, which lends its properties from a maximum
entropy state of a Tonks gas. Using this connection we show that spatial
correlations between Rydberg atoms are controlled by the number of atoms
contained within a blockade volume. For a small number the system favors a
disordered arrangement of Rydberg atoms, whereas in the opposite limit Rydberg
atoms tend to arrange in an increasingly ordered configuration. We argue that
this is an entropic effect which is observable in current experiments.
