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Physical Review A

Persistent currents in cold atoms

Jian-Jun Song, Bradley A. Foreman

Published Sep 4, 2009• 15 citations
Publisher

Abstract

We propose a scheme to realize persistent mass and spin currents in cold atoms using optically generated gauge potentials. Two degenerate dark states can be created by coupling ``tripod'' atoms to three Laguerre-Gaussian laser beams carrying relative orbital angular momentum. Atoms in these dark states feel opposite effective magnetic fields of the flux-tube type and circulate in opposite directions around the axis of the tube, generating persistent mass currents in the system. Persistent spin currents can also be generated, with or without spin polarization.

DOI
10.1103/physreva.80.033602

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We propose a scheme to realize persistent mass and spin currents in cold atoms using optically generated gauge potentials. Two degenerate dark states can be created by coupling ``tripod'' atoms to three Laguerre-Gaussian laser beams carrying relative orbital angular momentum. Atoms in these dark states feel opposite effective magnetic fields of the flux-tube type and circulate in opposite directions around the axis of the tube, generating persistent mass currents in the system. Persistent spin currents can also be generated, with or without spin polarization.
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We show that the adiabatic motion of ultracold, multilevel atoms in spatially varying laser fields can give rise to effective non-Abelian gauge fields if degenerate adiabatic eigenstates of the atom-laser interaction exist. A pair of such degenerate dark states emerges, e.g., if laser fields couple three internal states of an atom to a fourth common one under pairwise two-photon-resonance conditions. For this so-called tripod scheme we derive general conditions for truly non-Abelian gauge potentials and discuss special examples. In particular we show that using orthogonal laser beams with orbital angular momentum an effective magnetic field can be generated that has a monopole component.
Using laser beams to generate gauge potentials in cold atoms provides an opportunity to study interesting quantum phenomena in condensed matter physics. In this thesis, a scheme to manipulate atomic wave functions and generate spin current in a trapped BEC—whose ground state can be freely controlled to be a vortex or non-vortex state—has been studied and discussed. Laguerre-Gaussian beams, which carry non-zero orbital angular momentum, can be used to create coupling between the 23S1 and 23P1 states in 4He atoms. Wavefunctions in atoms interacting with these beams have been expressed in new dressed states. For atoms moving adiabatically, the coupling among the dressed states can be neglected through adiabatic approximation and centre of mass of atoms would experience both effective vector potential and effective scalar potential. The eigenenergies and wavefunctions of a single atom have been calculated by computational methods. In addition, the ground-state energies, wavefunctions and spin current of many-body systems, in which the non-linear Gross-Pitaevskii particle-particle interaction is included, have been found. Comparing the result of a many-body system and those in the single particle system, the wavefunctions of the many-body system spread over a larger region than that in the single particle system, causing a significant difference in their spin current density. Also, the ground-state energy per particle in the many-body system is higher than that in the single particle system.
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 examine the use of Laguerre-Gaussian (LG) laser beams in the tripod scheme to create the coupling between spin and orbital angular momentum in a Bose–Einstein condensate (BEC) confined by a harmonic trap. We derive and solve the Gross-Pitaeskii equation numerically to obtain the density distribution of the BEC. Orbital angular momentum from the LG beam is transferred to the BEC, inducing the formation of a giant vortex at the BEC’s centre. The macroscopic flow of spinor BEC around the vortex leads to a persistent spin current in the BEC.