Light can carry two kinds of angular momentum: Spin angular momentum (SAM) associated with its polarization and orbital angular momentum (OAM) associated with its spatial mode [1, 2]. The coupling of optical SAM to atoms has been known for over a century  and been verified in numerous experiments. Light beams with OAM, however, have only recently been created [4, 5] and, in the past decade, the transfer of this angular momentum has been demonstrated by the rotation of macroscopic objects [6-8]. No experiments, however, have demonstrated the quantized transfer of the OAM of a photon to an atom. Here we directly demonstrate the coherent, quantized transfer of the OAM of photons to atoms in a Bose-Einstein condensate (BEC). Using a 2-photon stimulated Raman process, similar to Bragg diffraction , but with Laguerre-Gaussian beams carrying OAM of ? per photon, we generate an atomic vortex state, corresponding to a cloud of atoms rotating with angular momentum per atom quantized in units of ?. Furthermore, we demonstrate that the process is coherent by creating superpositions of different vortex states where the relative phase between the states is determined by the relative phases of the optical fields. As every atom in a BEC is essentially identical, our results demonstrate that the OAM is transferred in quantized units of ?. Our process represents both a new and well-controlled way of creating a vortex state in a BEC and a new tool for the coherent control of the OAM of atomic samples, complementing existing tools for linear momentum and internal angular momentum.
Pub Type: Journals
BEC, Laguerre-Gaussian, orbital angular momentum, photon-atom interaction, rotational states, superposition, vortex, vortices