  The topological insulator is a fundamentally new phase of matter, with the
striking property that the conduction of electrons occurs only on its surface,
not within the bulk, and that conduction is topologically protected.
Topological protection, the total lack of scattering of electron waves by
disorder, is perhaps the most fascinating and technologically important aspect
of this material: it provides robustness that is otherwise known only for
superconductors. However, unlike superconductivity and the quantum Hall effect,
which necessitate low temperatures or magnetic fields, the immunity to disorder
of topological insulators occurs at room temperature and without any external
magnetic field. For this reason, topological protection is predicted to have
wide-ranging applications in fault-tolerant quantum computing and spintronics.
Recently, a large theoretical effort has been directed towards bringing the
concept into the domain of photonics: achieving topological protection of light
at optical frequencies. Besides the interesting new physics involved, photonic
topological insulators hold the promise for applications in optical isolation
and robust photon transport. Here, we theoretically propose and experimentally
demonstrate the first photonic topological insulator: a photonic lattice
exhibiting topologically protected transport on the lattice edges, without the
need for any external field. The system is composed of an array of helical
waveguides, evanescently coupled to one another, and arranged in a
graphene-like honeycomb lattice. The chirality of the waveguides results in
scatter-free, one-way edge states that are topologically protected from
scattering.
