Storage and manipulation of orbital angular momentum of light in a cold atomic ensemble

The paper reports on the storage of superposition of orbital angular momentum (OAM) states, as well as its manipulation through an applied transverse magnetic field. In the experiment , Zeeman sublevels of the degenerate two-level system is used. In another series of experiments,an external dc magnetic field is applied, nearly orthogonal to the plane defined by the incident beams. In this case,a series of collapses and revivals of the stored Zeeman coherence is observed. These observations demonstrate that superposition of OAM states can be stored into the atomic ensemble, and also the reversible transfer of OAM between light and atoms. Moreover, it also shows that the stored OAM can be manipulated with an external magnetic field.