  Maxwell's equations, formulated 150 years ago, ultimately describe properties
of light, from classical electromagnetism to quantum and relativistic aspects.
The latter ones result in remarkable geometric and topological phenomena
related to the spin-1 massless nature of photons. By analyzing fundamental spin
properties of Maxwell waves, we show that free-space light exhibits an
intrinsic quantum spin Hall effect, i.e., surface modes with strong
spin-momentum locking. These modes are evanescent waves that form, e.g.,
surface plasmon-polaritons at vacuum-metal interfaces. Our findings illuminate
the unusual transverse spin in evanescent waves and explain recent experiments
demonstrating the transverse spin-direction locking in the excitation of
surface optical modes. This deepens our understanding of Maxwell's theory,
reveals analogies with topological insulators for electrons, and offers
applications for robust spin-directional optical interfaces.
