Editorial Introduction to JSTQE Issue on Photonics for Synthetic Dimension and Topological Insulators

The seven papers in this special on photonics for synthetic dimension and topological insulators. The idea of synthetic dimensions was originally introduced in the context of atomic physics, where under a suitably designed optical dressing the Zeeman sublevels of a given electronic state can be seen as an additional spatial dimension; the higher the spin, the larger the number of sites in this synthetic dimension. In photonics, this idea can be pushed much further using multimode resonators which host a huge number of almost equispaced modes such as ring-resonators. In a lattice of such resonators, the physical dimensions are in this way supplemented by a synthetic dimension corresponding to the mode index, which can be naturally addressed via frequency-multiplexing techniques. Recent work has pointed out that tunnel-like coupling along the synthetic dimension can be generated by means of a suitable temporal modulation of the optical properties of the resonator, e.g. via a strong optical pump beam driving the optical nonlinearity of the resonator medium. On the other hand, topological photonics has been a hot topic in the theoretical and applied physics communities, introducing concepts such as protected topological states and robust optical transport channels that maintain their functions even in the presence of significant disorder and imperfections.