  Many-body systems relaxing to equilibrium can exhibit complex dynamics even
if their steady state is trivial. At low temperatures or high densities their
evolution is often dominated by steric hindrances affecting particle motion
[1,2,3]. Local rearrangements are highly constrained, giving rise to collective
- and often slow - relaxation.This dynamics can be difficult to analyse from
first principles, but the essential physical ingredients are captured by
idealized lattice models with so- called kinetic constraints [4]. Here we
experimentally realize a many-body system exhibiting manifest kinetic
constraints and measure its dynamical properties. In the cold Rydberg gas used
in our experiments, the nature of the constraints can be tailored through the
detuning of the excitation lasers from resonance [5,6,7,8], which controls
whether the system undergoes correlated or anti- correlated dynamics. Our
results confirm recent theoretical predictions [5,6], and highlight the analogy
between the dynamics of interacting Rydberg gases and that of soft-matter
systems.
