  We discuss the quasi 1-D scattering of two counter-propagating, dark-state
polaritons (DSP), each containing a single excitation. DSPs are formed from
photons in media with electromagnetically induced transparency and are
associated with ultra-slow group velocities. State-dependent elastic collisions
of atoms at the same lattice site lead to a nonlinear interaction. It is shown
that the scattering process in a deep optical lattice filled by cold atoms
generates a large and homogeneous conditional phase shift between two
individual polaritons. The latter has potential applications for a photonic
phase gate. The quasi 1-D scattering problem is solved analytically and the
influence of degrading processes such as dephasing due to collisions with
ground-state atoms is discussed.
