Superradiant exchange of orbital angular momentum between light and cold atoms

An analysis of superradiant scattering of light by ultracold atoms is presented in which the pump and scattered light consist of hollow counterrotating Laguerre-Gaussian beams possessing orbital angular momentum. It is shown that the interaction between the Laguerre-Gaussian beams and the atoms can give rise to a superradiant instability in which orbital angular momentum is transferred to the atoms and the probe field is amplified. In the case where the atoms constitute a dilute Bose-Einstein condensate (BEC) that is trapped in a one-dimensional ring geometry, the transfer of orbital angular momentum to the BEC during the instability means that the BEC spontaneously develops rotation in the ring.