Hollow-core photonic-crystal waveguides filled with cold atoms can support giant optical nonlinearities
through nondispersive propagation of light tightly confined in the transverse direction. Here we explore
electromagnetically induced transparency is such structures, considering a pair of counterpropagating weak
quantum fields in the medium of coherently driven atoms in the ladder configuration. Strong dipole-dipole
interactions between optically excited, polarized Rydberg states of the atoms translate into a nonlocal and large dispersive
interaction between the two quantum fields. This can be used to attain a spatially homogeneous conditional phase shift
of π for two single-photon pulses, realizing a deterministic photonic phase gate.

Phys. Rev. A 83, 033806 (2011)