Entropic enhancement of spatial correlations in a laser-driven Rydberg   gas
  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. 
C. Ates and I. Lesanovsky
physics.atom-ph cond-mat.stat-mech