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Abstract: |
The
interaction of photons with atoms inevitably involves the exchange
of momentum. The transfer of spin angular momentum from light to an
atom has been known for almost a century and can be used, very
effectively, to change the internal state of an atom. Similarly, the
past couple of decades have witnessed a tremendous growth in the use
of light to control the center-of-mass motion of atoms. For example,
the linear momentum of light can be utilized to laser cool and trap
atoms. Light, in addition to carrying spin and linear momentum, can
also carry orbital angular momentum. The orbital angular momentum of
light, which is associated with its spatial mode, has been used to
rotate macroscopic objects; however, the rotation of atoms due to
the orbital angular momentum of photons has not been directly
observed.
I will
describe experiments in which we demonstrate [1] 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, which carry orbital angular momentum, we
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. Furthermore, we create vortices of charge 2 by
transferring to each atom the orbital angular momentum of two
photons, each with orbital angular momentum h.
We
subsequently use this technique to generate rotational flow of a BEC
confined in a toroidal shaped trap. The toridal trap is formed by
using a blue detuned laser beam to exclude atoms from the central
region of an elliptically shaped magnetic trap. We measure that the
flow of atoms persists for up to 10 seconds, which we interpret as
the first direct evidence of a persistent current in a superfluid
Bose gas. Stable flow was only possible in the multiply-connected
geometry of the toriodal trap, and was observed for a BEC fraction
as small as 15%. We also observed flow with higher angular momentum
(winding number), and its splitting into singly-charged vortices
when the trap topology was changed from multiply- to
simply-connected.
References:
[1] M. F.
Andersen et al., Phys. Rev. Lett. 97, 170406 (2006). |