  We report the experimental observation of the rotation of the polarization
plane of light propagating in a gas of fast-spinning molecules (molecular
super-rotors). In the observed effect, related to Fermi's prediction of
"polarization drag" by a rotating medium, the vector of linear polarization
tilts in the direction of molecular rotation due to the rotation-induced
difference in the refractive indices for the left and right circularly
polarized components. We use an optical centrifuge to bring the molecules in a
gas sample to ultrafast unidirectional rotation and measure the polarization
drag angles of the order of 0.2 milliradians in a number of gases under ambient
conditions. We demonstrate an all-optical control of the drag magnitude and
direction, and investigate the robustness of the mechanical Faraday effect with
respect to molecular collisions.
