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Science

Atom Beams Split by Gentle Persuasion

Robert Pool

Published Feb 25, 1994• 13 citations
Publisher

Abstract

Two different research teams have taken a big step toward atom interferometry. They have succeeded in splitting atomic beams by using atoms in spin states that neither absorb nor reemit laser light. By proper adjustment of experimental conditions, atoms are changed from one spin state to another, without passing through the intermediary excited state. The atoms in essence absorb momentum from the laser photons, without absorption or emission of photons. The change in momentum deflects atoms in the proper spin state.

DOI
10.1126/science.263.5150.1092

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Two different research teams have taken a big step toward atom interferometry. They have succeeded in splitting atomic beams by using atoms in spin states that neither absorb nor reemit laser light. By proper adjustment of experimental conditions, atoms are changed from one spin state to another, without passing through the intermediary excited state. The atoms in essence absorb momentum from the laser photons, without absorption or emission of photons. The change in momentum deflects atoms in the proper spin state.
A golden rule of laser technology is that a majority of the atoms in the lasing medium must be in the excited, light-emitting state. But two teams of physicists have managed to break this rule: They have obtained light from just a few excited atoms by preventing the others from absorbing photons. The trick was to use a second laser to alter the atomic energy levels, creating two different paths by which the atoms could absorb the light. The paths competed, and absorption was stymied. See sidebar (p. 738).
We have demonstrated a novel beam splitter for atoms. The J=1 to J=0 system in metastable helium has two independent ``dark'' states which are radiatively stable. By preparing atoms in the two ``dark'' states of the 23{2}^{3}23S1{\mathit{S}}_{1}S1​ level and passing them through a sequence of partially overlapping laser beams tuned to the 23{2}^{3}23${\mathit{S}}_{1\mathrm{\ensuremath{-}}}$22{2}^{2}22P0{\mathit{P}}_{0}P0​ transition, we cause the atoms in one ``dark'' state to be deflected by integral multiples of 2\ensuremath{\Elzxh}k, while the atoms in the other ``dark'' state are undeflected. The excited 23{2}^{3}23P0{\mathit{P}}_{0}P0​ state is not populated, so the process is unaffected by spontaneous emission. We have observed up to 90% coherent momentum transfer with 4\ensuremath{\Elzxh}k deflection. We conclude by describing how one might construct an atom interferometer based on these ``dark state'' beam splitters.
We have observed momentum transfer from a standing-wave light field to an atomic beam. The atomic beam is split symmetrically into two peaks whose separation increases with field strength. The short interaction time ensures that this deflection is due to induced forces; these are described using a semiclassical dressed-atom treatment, which gives good agreement with the data. In addition to the splitting, we have observed diffraction of the atomic beam due to the exchange of even numbers of photons with the field.
We demonstrate a new technique for the mechanical manipulation of atoms with light that may be used to deflect or split an atomic beam. This technique depends on the existence of an internal superposition state of the atom that is 'dark' to resonant excitation by a particular light field. An atom in a dark state may adiabatically follow a slowly varying light field in such a way that both the internal state and the atom's momentum are changed. Because the dark state never absorbs or fluoresces, the atomic coherence, necessary for atom interferometry, is preserved. We use laser-cooled Cs atoms to demonstrate the transfer of 8 photon momenta from the slowly varying laser field to the atom.
We study atomic-beam deflection by adiabatic passage between Zeeman ground levels via Raman transitions induced by counterpropagating ${\mathrm{\ensuremath{\sigma}}}^{\ifmmode\pm\else\textpm\fi{}}$-polarized lasers. We show that complete population transfer between the ground states can be achieved, which corresponds to the scattering of the atomic wave packet into a single final momentum state by absorption and induced emission of laser photons. Although the lasers can be resonant, the excited state(s) are never populated during the adiabatic transfer, which suppresses the effects of spontaneous emission and preserves the coherence of the atomic wave function. This scheme has attractive features as a beam splitter and mirror for atomic interferometry.
Optical interference is proof of the wave nature of light and matter–wave interferometers have also been shown to work for electrons and neutrons (by exploiting the de Broglie wavelength of the particles). Now a working interferometer for atoms has been demonstrated at the Massachusetts Institute of Technology(MIT) for the first time, in the sense that it uses “atomic beam splitters” to separate and recombine atomic beams (David W Keith et al. Phys. Rev. Lett. (1991) 66 2693). A simpler experiment at Konstanz University, Germany (O Carnal and J Mlynek Phys. Rev. Lett. (1991) 66 2689) has also demonstrated interference phenomena in atomic beams but a detailed description of the apparatus is not yet published. Atom interferometers will some day be useful in precision metrology, mass tomography, precision gyroscopes, general relativity, fundamental quantum mechanics and atomic physics.