  Topological insulators (TIs) are a striking example of materials in which
topological invariants are manifested in robustness against perturbations.
Their most prominent feature is the emergence of topological edge states with
reduced dimension at the boundary between areas with distinct topological
invariants. The observable physical effect is unidirectional robust transport,
unaffected by defects or disorder. TIs were originally observed in the integer
quantum Hall effect for fermionic systems of correlated electrons. However,
during the past decade the concepts of topological physics have been introduced
into numerous fields beyond condensed matter, ranging from microwaves and
photonic systems to cold atoms, acoustics and even mechanics. Recently, TIs
were proposed in exciton-polariton systems organized as honeycomb lattices,
under the influence of a magnetic field. Topological phenomena in polaritons
are fundamentally different from all topological effects demonstrated
experimentally thus far: exciton-polaritons are part-light part-matter
quasiparticles emerging from the strong coupling of quantum well excitons and
cavity photons. Here, we demonstrate experimentally the first exciton-polariton
TI. This constitutes the first symbiotic light-matter TIs. Our polariton
lattice is excited non-resonantly, and the chiral topological polariton edge
mode is populated by a polariton condensation mechanism. We image real- and
Fourier-space to measure photoluminescence, and demonstrate that the
topological edge mode avoids defects, and that the propagation direction of the
mode can be reversed by inverting the applied magnetic field. Our
exciton-polariton TI paves the way for a variety of new topological phenomena,
as they involve light-matter interaction, gain, and perhaps most importantly -
exciton-polaritons interact with one another as a nonlinear many-body system.
