Observation of topologically protected bound states in photonic quantum walks

Topological phenomena in physical systems are a direct consequence of the topology of the underlying wave-functions and are robust against perturbations; for example, the Hall conductance induced by the integer quantum Hall effect is very precisely quantifiedindependent of intrinsic (e.g. impurities) or extrinsic (size, shape) characteristics of the studied samples. The study of these phenomena in condensed matter physics is often hard, due to the rapid decoherence of electronswhich are strongly coupled to their environmentand the difficulty of directly imaging electronic wave functions. Artificial quantum systems, such as in quantum optics, can in contrast be precisely controlled and provide an ideal setting for realizing, manipulating, and probing topological phases. The study of topological phases then does not have to remain limited to static or quasi-static/adiabatic situations, it can be extended to periodically driven systems, which have recently been proposed to also exhibit topological behaviors [1]