The realization of strong nonlinear interactions between individual
light quanta (photons) is a long-standing goal in optical science and
engineering, being of both fundamental and technological significance.
In conventional optical materials, the nonlinearity at light
powers corresponding to single photons is negligibly weak. We
demonstrate a medium that is nonlinear at the level of individual
quanta, exhibiting strong absorption of photon pairs while remaining
transparent to single photons. The quantum nonlinearity is
obtained by coherently coupling slowly propagating photons to
strongly interacting atomic Rydberg states in a cold, dense
atomic gas. Novel non-equilibrium quantum phenomena in such a system,
including the realization
of strongly correlated many-body states of light, will be discussed.
Author entry (protected)