  Coulomb interaction has important consequences on the physics of quantum spin
Hall edge states, weakening the topological protection via two-particle
scattering and renormalizing both the velocity and charge of collective plasmon
modes compared to that of free electrons. Despite these effects, interactions
remain difficult to quantify. We propose here simple and robust edge resonator
geometries to characterize Coulomb interaction by means of high-frequency
measurements. They rely on a transmission line approach, and take advantage of
the impedance mismatch between the edge states and their microwave environment.
