We present a quantum theory of slow light beyond the weak probe pulse
approximation. By reduction of the full Hamiltonian of the system to an
effective Hamiltonian for a single quantum field we demonstrate that the
concept of dark-state polaritons can be introduced even if the linearized
approach is no longer valid. The developed approach allows us to study the
evolution of non-classical quantum states of the polariton field.Comment: 9 pages, 4 figure
