Abstract

Light beams carrying orbital angular momentum (OAM) have attracted an enormous recent interest owing to the possibility of encoding quantum information in a multidimensional state space [1]. We report the storage of superposition of OAM states, as well as its manipulation through an applied transverse magnetic field. In the experiment we used the Zeeman sublevels of the degenerate two-level system associated with the cycling transition 6S_1/2, F=3-6P_3/2, F=2 of a cold cesium sample. Two incident writing beams, labelled as W and W, with opposite circular polarization are incident on the cold ensemble forming a small angle. We consider the case where the beam W" can carry a superposition of OAM states, associated with the Laguerre-Gaussian modes with different topological charge m, i. e., $L{G}_0^m$. The writing beams therefore excite a ground-state coherence grating into the atomic ensemble, transferring their relative phase information to the induced Zeeman coherence. After the switching off of the writing beams the induced grating is probed by a reading beam R, which is counter-propagating to the writing beam W and has a circular polarization opposite to that beam. In the continuous wave (cw) excitation of the ensemble, this corresponds to the well-known backward FWM configuration. The retrieved signal D corresponds to the Bragg diffraction of beam R into the stored coherence grating and is counter-propagating to the write beam W. The images (a) and (b) of Fig. 1, show respectively the intensity profile corresponding to an incident writing beam in the mode W'=$L{G}_0^1$ and the diffracted retrieved beam after 1.0 μs storage time. In order to measure the topological charge of the retrieved beam we superpose with the incident writing beam W'a Gaussian beam which will generate the necessary reference wave. Thus, the. image (c) corresponds to the retrieved beam associated with an incident superposition W' =$L{G}_0^0$+$L{G}_0^1$ We thus have verified that the incident and the retrieved beams have the same single topological charge.

© 2009 IEEE