  Electromagnetic (EM) waves propagating through an inhomogeneous medium
inevitably scatter whenever electromagnetic properties of the medium change on
the scale of a single wavelength. This fundamental phenomenon constrains how
optical structures are designed and interfaced with each other. Recent
theoretical work indicates that electromagnetic structures collectively known
as photonic topological insulators (PTIs) can be employed to overcome this
fundamental limitation, thereby paving the way for ultra-compact photonic
structures that no longer have to be wavelength-scale smooth. Here we present
the first experimental demonstration of a photonic delay line based on
topologically protected surface electromagnetic waves (TPSWs) between two PTIs
which are the EM counterparts of the quantum spin-Hall topological insulators
in condensed matter. Unlike conventional guided EM waves that do not benefit
from topological protection, TPSWs are shown to experience multi-wavelength
reflection-free time delays when detoured around sharply-curved paths, thus
offering a unique paradigm for compact and efficient wave buffers and other
devices.
