Momentum-Space Topological Effects of Nonreciprocity

The connection between topology and nonreciprocity in photonic systems is reviewed. Topological properties such as Chern number, and momentum-space properties such as Berry phase and Berry connection, are used to explain backscattering immune edge states and their topological protection. We consider several examples to illustrate the role of momentum-space topology on wave propagation, and discuss recent magnet-less approaches.