  Much of the recent enthusiasm directed towards topological insulators as a
new state of matter is motivated by their hallmark feature of protected chiral
edge states. In fermionic systems, Kramers degeneracy gives rise to these
entities in the presence of time-reversal symmetry (TRS). In contrast, bosonic
systems obeying TRS are generally assumed to be fundamentally precluded from
supporting edge states. In this work, we dispel this perception and
experimentally demonstrate counter-propagating chiral states at the edge of a
time-reversal-symmetric photonic waveguide structure. The pivotal step in our
approach is encoding the effective spin of the propagating states as a degree
of freedom of the underlying waveguide lattice, such that our photonic
topological insulator is characterised by a $\mathbb{Z}_2$-type invariant. Our
findings allow for fermionic properties to be harnessed in bosonic systems,
thereby opening new avenues for topological physics in photonics as well as
acoustics, mechanics and even matter waves.
