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Quantum Optics Journal of the European Optical Society Part B

Selection rules and centre-of-mass motion of ultracold atoms

S. J. van Enk

Published Oct 1, 1994• 47 citations
Publisher

Abstract

In recent years much attention has been paid to the quantized evolution of the centre-of-mass momentum and position of ultracold atoms in light fields. We consider the effects resulting from the quantization of the external angular momentum variables. We investigate how spin and orbital angular momentum of light are transferred to internal and external angular momentum of an atom in dipole and quadrupole transitions.

DOI
10.1088/0954-8998/6/5/007

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In recent years much attention has been paid to the quantized evolution of the centre-of-mass momentum and position of ultracold atoms in light fields. We consider the effects resulting from the quantization of the external angular momentum variables. We investigate how spin and orbital angular momentum of light are transferred to internal and external angular momentum of an atom in dipole and quadrupole transitions.
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In the interaction of molecules with light endowed with orbital angular momentum, an exchange of orbital angular momentum in an electric dipole transition occurs only between the light and the center of mass motion; i.e., internal "electronic-type" motion does not participate in any exchange of orbital angular momentum in a dipole transition. A quadrupole transition is the lowest electric multipolar process in which an exchange of orbital angular momentum can occur between the light, the internal motion, and the center of mass motion. This rules out experiments seeking to observe exchange of orbital angular momentum between light beams and the internal motion in electric dipole transitions.
In the preceding chapters, we discussed the manipulation and observation of atomic variables associated with the internal degrees of freedom. A complete separation between internal and external degrees of freedom, like position and momentum, is not always possible, however. It therefore appears appropriate to include a discussion of the mechanical effects of light, which have received increasing attention in recent years. The transfer of momentum between photons and atoms, first observed more than sixty years ago, had to wait for the wide availability of lasers to become a useful experimental tool. In particular the cooling of atoms and atomic ions to very low temperatures has found numerous applications. Light-induced forces That light exerts mechanical forces on massive particles like atoms may appear surprising. To motivate the existence of such an effect two different approaches are possible. The first approach considers the light as a collection of photons that carry, apart from energy and angular momentum, linear momentum as well. Photons interacting with atoms can therefore change the momentum of the atoms. The second approach considers light as a wave, i.e., an inhomogeneous electromagnetic field interacting with the atomic dipole moment. Both approaches provide a possible description for the numerous phenomena that can occur in this context, but in many situations, one of them turns out to be more intuitive or more useful for calculations than the other.