The hallmark property of two-dimensional topological insulators is robustness of quantized electronic transport of charge and energy against disorder in the underlying lattice1. That robustness arises from the fact that, in the topological bandgap, such transport can occur only along the edge states, which are immune to backscattering owing to topological protection. However, for sufficiently strong disorder, this bandgap closes and the system as a whole becomes topologically trivial: all states are localized and all transport vanishes in accordance with Anderson localization2,3. The recent suggestion4 that the reverse transition can occur was therefore surprising. In so-called topological Anderson insulators, it has been predicted4 that the emergence of protected edge states and quantized transport can be induced, rather than inhibited, by the addition of sufficient disorder to a topologically trivial insulator. Here we report the experimental demonstration of a photonic topological Anderson insulator. Our experiments are carried out in an array of helical evanescently coupled waveguides in a honeycomb geometry with detuned sublattices. Adding on-site disorder in the form of random variations in the refractive index of the waveguides drives the system from a trivial phase into a topological one. This manifestation of topological Anderson insulator physics shows experimentally that disorder can enhance transport rather than arrest it.
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A.S. and M.S. thank the German-Israeli DIP (project BL 574/13-1). A.S. acknowledges funding from the German Research Foundation (project SZ 276/9-1). M.S. thanks the European Research Council for financial support. N.L. acknowledges financial support from the European Research Council under the European Union Horizon 2020 Research and Innovation Programme (grant agreement number 639172), from the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme (FP7/2007-2013) under REA grant agreement number 631696 and from the Israeli Center of Research Excellence (I-CORE) Circle of Light, funded by the Israeli Science Foundation. M.C.R. acknowledges support from the National Science Foundation under grant number DMS-1620422, as well as the Sloan (FG-2016-6418) and Kaufman (KA2017-91788) foundations. P.T. is supported by an NRC postdoctoral fellowship. The authors acknowledge the University of Maryland supercomputing resources made available for conducting the research reported in this paper.
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Nature Reviews Physics (2019)