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Topological insulators are a new phase of matter, with the striking property that conduction of electrons occurs
only on the surface. In two dimensions, surface electrons in topological insulators do not scatter despite defects
and disorder, providing robustness akin to superconductors. Topological insulators are predicted to have wideranging
applications in fault-tolerant quantum computing and spintronics. Recently, large theoretical efforts were
directed towards achieving topological insulation for electromagnetic waves. One-dimensional systems with
topological edge states have been demonstrated, but these states are zero-dimensional, and therefore exhibit no
transport properties. Topological protection of microwaves has been observed using a mechanism similar to
the quantum Hall effect, by placing a gyromagnetic photonic crystal in an external magnetic field. However,
since magnetic effects are very weak at optical frequencies, realizing photonic topological insulators with scatterfree
edge states requires a fundamentally different mechanism - one that is free of magnetic fields. Recently, a
number of proposals for photonic topological transport have been put forward. Specifically, one suggested
temporally modulating a photonic crystal, thus breaking time-reversal symmetry and inducing one-way edge
states. This is in the spirit of the proposed Floquet topological insulators, where temporal variations in solidstate
systems induce topological edge states. Here, we propose and experimentally demonstrate the first external
field-free photonic topological insulator with scatter-free edge transport: a photonic lattice exhibiting topologically
protected transport of visible light on the lattice edges. Our system is composed of an array of evanescently coupled
helical waveguides arranged in a graphene-like honeycomb lattice. Paraxial diffraction of light is described by
a Schrödinger equation where the propagation coordinate acts as ‘time’. Thus the waveguides' helicity breaks zreversal
symmetry in the sense akin to Floquet Topological Insulators. This structure results in scatter-free, oneway
edge states that are topologically protected from scattering.
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