Possible Realization of Directional Optical Waveguides in Photonic   Crystals with Broken Time-Reversal Symmetry
  We show how in principle to construct analogs of quantum Hall edge states in "photonic crystals" made with non-reciprocal (Faraday-effect) media. These form "one-way waveguides" that allow electromagnetic energy to flow in one direction only. 
F. D. M. Haldane and S. Raghu