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Topologica

Vortices and Persistent Currents: Rotating a Bose-Einstein Condensate Using Photons with Orbital Angular Momentum

Kristian Helmerson, Mikkel F. Andersen, Pierre Cladé, Vasant Natarajan A, William D. Phillips, Anand K. Ramanathan, Changhyun Ryu, Alipasha Vaziri

Published Jan 1, 2009• 6 citations
Publisher

Abstract

We describe the coherent transfer of the orbital angular momentum of a photon to an atom in quantized units of ℏ, 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 toriodal 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.

DOI
10.3731/topologica.2.002

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We describe the coherent transfer of the orbital angular momentum of a photon to an atom in quantized units of ℏ, 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 toriodal 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.
We demonstrate the coherent transfer of the orbital angular momentum of a photon to an atom in quantized units of h, using a 2-photon stimulated Raman process with Laguerre-Gaussian beams to generate an atomic vortex state in a Bose-Einstein condensate 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. Furthermore, we create vortices of charge 2 by transferring to each atom the orbital angular momentum of two photons. We subsequently use our technique to induce rotation of the condensate confined in an asymmetric, ring-shaped hybrid optical and magnetic trap. We observe the cloud rotating for up to 13 seconds, due to the superfluid character of the condensate.
We demonstrate the coherent transfer of the orbital angular momentum of a photon to an atom in quantized units of variant Planck's over 2pi, using a 2-photon stimulated Raman process with Laguerre-Gaussian beams to generate an atomic vortex state in a Bose-Einstein condensate 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. Furthermore, we create vortices of charge 2 by transferring to each atom the orbital angular momentum of two photons.
We demonstrate the coherent, quantized transfer of the orbital angular momentum of light to a Bose-Einstein condensate of sodium atoms by means of a stimulated, two-photon transition between the static, non-rotating BEC and a rotating state moving with the linear momentum of two photons. By reversing the transfer of linear momentum, without any angular momentum, we create either vortex states (in a simply connected trap) or persistent atom currents (in a toroidal trap). The persistent current in the toroidal trap lasts about 20 times longer than a vortex under similar conditions in the simply connected trap.
We have observed the persistent flow of Bose-condensed atoms in a toroidal trap. The flow persists without decay for up to 10 s, and was initiated by transferring one unit, ħ, of the orbital angular momentum from Laguerre-Gaussian photons to each atom. Stable flow was only possible when the trap was multiply-connected, and was observed with a BEC fraction as small as 15%. We also created flow with two units of angular momentum, and observed its splitting into two singly-charged vortices when the trap geometry was changed from multiply- to simply-connected.
We present experimental results on the coherent transfer of orbital angular momentum (OAM) from optical fields to the center-of-mass motion of a Bose-Einstein condensate (BEC), using an approach which results in negligible linear momentum change. Two collinear optical beams of differing OAM are used to couple discrete sublevels of the atomic ground state via a stimulated Raman process. We demonstrate several of the possible optical field configurations which place the BEC into various spinor and vortex states. We also show how this approach can be used to create coherent superpositions of these states and use the interference patterns of such superpositions to confirm the presence of quantized vortices.