Measurement of the Rotational Frequency Shift Imparted to a Rotating Light Beam Possessing Orbital Angular Momentum

We observe the frequency shift, $l\Omega$, imparted to a mm-wave beam with an orbital angular momentum of $lħ$ per photon, when the beam is rotated at angular frequency $\Omega$. We show that this shift, and those found in a number of experiments on the rotation of circularly polarized beams, are special cases of the rotational frequency shift recently predicted by Bialynicki-Birula and Bialynicka-Birula. The measurement also explicitly confirms a theoretical prediction by Nienhuis.