Effective magnetic fields in degenerate atomic gases induced by light beams with orbital angular momenta

We investigate the influence of two resonant laser beams on the mechanical properties of degenerate atomic gases. The control and probe beams of light are considered to have orbital angular momenta (OAM) and act on the three-level atoms in the electromagnetically induced transparency configuration. The theory is based on the explicit analysis of the quantum dynamics of cold atoms coupled with two laser beams. Using the adiabatic approximation, we obtain an effective equation of motion for the atoms driven to the dark state. The equation contains a vector-potential-type interaction as well as an effective trapping potential. The effective magnetic field is shown to be oriented along the propagation direction of the control and probe beams containing OAM. Its spatial profile can be controlled by choosing proper laser beams. We demonstrate how to generate a constant effective magnetic field, as well as a field exhibiting a radial distance dependence. The resulting effective magnetic field can be concentrated within a region where the effective trapping potential holds the atoms. The estimated magnetic length can be considerably smaller than the size of the atomic cloud. © 2005 The American Physical Society.