Exceptional points and stopping of light

We discuss exceptional point (EP) degeneracies in linear PT-symmetric systems, focusing specifically on
waveguides. Here EP is the point, where two eigenvalues and respective eigenvectors merge forming the
Jordan block algebraic structure, and PT-symmetry features wave systems, where loss is compensated by
gain in a parity-symmetric manner. We start with a general introduction describing the mathematical
structure and physical significance of spectral properties related to EPs. As the central application, we
show that vanishing of a group velocity (i.e., full stop of a light pulse) in a waveguide is related to the EP
condition. This result is supported by numerical simulations for a pair of coupled waveguides, where the
gain/loss parameter is changed adiabatically in time