Copyright Institute of Experimental Physics, Vienna 2006

Laguerre-gaussian (LG) modes — light beams with twisted wave fronts — have attracted considerable attention in recent years owing to a fascinating property: they carry orbital angular momentum. LG modes are widely used for atom trapping and cooling and, more recently, have shown potential for use in quantum information processing. One phenomenon that intrigues researchers when atoms are placed in an LG beam is the rotational Doppler effect (RDE) — a shift of atomic energy levels associated with the rotation of the atom around the beam axis. However, the RDE is much smaller than the usual Doppler effect — which is related to movement in the axial and radial directions — making it difficult to observe.

An approach taken by Barreiro and colleagues from Uruguay and Brazil has enabled them to demonstrate the RDE in atoms resulting from an LG beam for the first time1. Using two LG beams with equal, but opposite, pitch, the team can ensure that the Doppler shifts due to the axial and the radial motion cancel out, leaving only the shift resulting from the RDE. Passing the two beams through a cloud of rubidium atoms, Barreiro et al. observe a reduction in the atomic absorption, the so-called Hanle electromagnetically induced transparency resonance. The RDE manifests itself as a broadening of this resonance. Increasing the pitch of the LG modes leads to an increase in the broadening. This latest work demonstrates another way in which light can be used for the manipulation of atomic motion.