Quantum spin Hall effect of light

A quantum twist on classical optics Interpreting recent experimental results of light interactions with matter shows that the classical Maxwell theory of light has intrinsic quantum spin Hall effect properties even in free space. Complex effects in condensed-matter systems can often find analogs in cleaner optical systems. Bliokh et al. argue that the optical systems exhibiting such complex phenomena should also be simpler (see the Perspective by Stone). Their theoretical study shows that free-space light has a nonzero topological spin Chern number and thus should have counterpropagating surface modes. Such modes are actually well known and can be described as evanescent modes of Maxwell equations. Science, this issue p. 1448; see also p. 1432 A theoretical study reveals that quantum effects may manifest in classical optical experiments. [Also see Perspective by Stone] 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—surface modes with strong spin-momentum locking. These modes are evanescent waves that form, for example, surface plasmon-polaritons at vacuum-metal interfaces. Our findings illuminate the unusual transverse spin in evanescent waves and explain recent experiments that have demonstrated 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.