  One-dimensional (1D) electron systems in the presence of Coulomb interaction
are described by Luttinger liquid theory. The strength of Coulomb interaction
in the Luttinger liquid, as parameterized by the Luttinger parameter K, is in
general difficult to measure. This is because K is usually hidden in powerlaw
dependencies of observables as a function of temperature or applied bias. We
propose a dynamical way to measure K on the basis of an electronic
time-of-flight experiment. We argue that the helical Luttinger liquid at the
edge of a 2D topological insulator constitutes a preeminently suited
realization of a 1D system to test our proposal. This is based on the
robustness of helical liquids against elastic backscattering in the presence of
time reversal symmetry.
