  Interfacing light and matter at the quantum level is at the heart of modern
atomic and optical physics and enables new quantum technologies involving the
manipulation of single photons and atoms. A prototypical atom-light interface
is electromagnetically induced transparency, in which quantum interference
gives rise to hybrid states of photons and atoms called dark-state polaritons.
We have observed individual dark-state polaritons as they propagate through an
ultracold atomic gas involving Rydberg states. Strong long-range interactions
between Rydberg atoms give rise to an effective interaction blockade for
dark-state polaritons, which results in large optical nonlinearities and
modified polariton number statistics. The observed statistical fluctuations
drop well below the quantum noise limit indicating that photon correlations
modified by the strong interactions have a significant back-action on the
Rydberg atom statistics.
