# PT Symmetry and Exceptional Points in Open Systems

Many of the new wave of nanophotonic structures are described in terms of PT symmetry. Many interesting effects have been studied and observed based on this formalism. Originally developed for complex potentials, which may still yield real eigenvalues for the dynamics, it has use in systems with gain and loss. As we will show, the usual application of PT in optics is to describe mean field theory do not adequately describe quantum noise. It is essentially quantum trajectory theory without the jumps. I will describe an old system, the driven Jaynes-Cummings model, as well as the more recent work on waveguide/fiber quantum electrodynamics. These studies did not use PT symmetry, but are perhaps clearer in this new light. I will also connect PT symmetry and non-Hermitian Hamiltonians with more traditional scattering theory.