  Recent advances in topologically robust waveguiding for electromagnetic
systems have presented opportunities for improving practical photonic and
microwave devices. To bring this rich area of physics within the reach of
application, it is critical for such systems to be interfaced with classical,
continuous waveguiding and transmission line technology. This Letter presents a
compact, highly efficient transition from a classical metallic transmission
line to a topologically nontrivial line wave emulating the quantum spin Hall
effect. A zero-gap antipodal slot line is used as the starting transmission
line, which is then coupled to the topological metasurface via a field matching
procedure. Additional modifications to the interface between the two structures
to eliminate unwanted edge coupling improves transmission further. A simulated
loss analysis isolates the effect of the transitions from the rest of the
structure, showing a loss contribution of only 2.1% per
classical-to-topological conversion. Using the transition, a quantitative
characterization of the robustness of common topologically protected devices is
presented. This design lays the foundation to integrate topologically robust
metasurface transmission lines to traditional systems, opening the door to
future uses of such structures in systems.
