We describe the coherent transfer of the orbital angular momentum of a photon to an atom in quantized units of a??, using a 2-photon stimulated Raman process with Laguerre-Gaussian beams to generate an atomic vortex state in a Bose-Einstein condensate (BEC) of sodium atoms. We show 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. We use this technique to generate circular flow of a BEC confined in a toroidal shaped trap. We measure that the flow of atoms persists for up to 10 seconds, which we interpret as the first evidence of persistent currents in a superfluid Bose gas.
Helmerson, K., Andersen, M., Ryu, C., Clade, P., Natarajan, V., Vaziri, A., & Phillips, W. D. (2007). Generating persistent currents states of atoms using orbital angular momentum of photons. Nuclear Physics A, 790(1-4), 705 - 712. https://doi.org/10.1016/j.nuclphysa.2007.03.019