  We propose a method for entangling a system of two-level atoms in photonic
crystals. The atoms are assumed to move in void regions of a photonic crystal.
The interaction between the atoms is mediated either via a defect mode or via
resonant dipole-dipole interaction. We show that these interactions can produce
pure entangled atomic states. We analyze the problem with parameters typical
for currently existing photonic crystals and Rydberg atoms. We show that the
atoms can emerge from photonic crystals in entangled states. Depending on the
linear dimensions of the crystal and on their velocity of the entangled atoms
can be separated by tens of centimeters.
