  In their study of superluminal propagation, rotary drag and surface
polaritons [Phys. Rev. A 96, 013848 and 049906(E) (2017)], Khan et al. consider
a four-level atomic arrangement with transitions in the optical domain. In
fact, the values they give to the parameters lead to a probe wavelength lying
in the decimeter band and we point out that, in such conditions, all their
results are irrelevant.
