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Physics Today

Experimenters Cool Helium Below Single-Photon Recoil Limit in Three Dimensions

Gloria B. Lubkin

Published Jan 1, 1996• 3 citations
Publisher

Abstract

A group at the Ecole Normale Superieure in Paris has placed a helium atom in a state that is going in six directions at once—a very confusing state for an atom. In such a superposition state, also called a dark state, the atom can't absorb light because the absorption amplitudes cancel out by destructive interference. The temperature corresponding to the velocity spread of each of the six wavepackets is a factor of 20 below the so-called recoil limit for laser-cooled atoms. The three-dimensional subrecoil experiment in helium was reported in the 4 December issue of Physical Review Letters by John La wall (now at the National Institute of Standards and Technology in Gaithersburg, Maryland), Si-mone Kulin, Bruno Saubamea, Nick Bigelow, Michèle Leduc and Claude Cohen-Tannoudji.

DOI
10.1063/1.2807458

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A group at the Ecole Normale Superieure in Paris has placed a helium atom in a state that is going in six directions at once—a very confusing state for an atom. In such a superposition state, also called a dark state, the atom can't absorb light because the absorption amplitudes cancel out by destructive interference. The temperature corresponding to the velocity spread of each of the six wavepackets is a factor of 20 below the so-called recoil limit for laser-cooled atoms. The three-dimensional subrecoil experiment in helium was reported in the 4 December issue of Physical Review Letters by John La wall (now at the National Institute of Standards and Technology in Gaithersburg, Maryland), Si-mone Kulin, Bruno Saubamea, Nick Bigelow, Michèle Leduc and Claude Cohen-Tannoudji.
We report the first observation of subrecoil laser cooling in three dimensions, where we use the method of velocity-selective coherent population trapping. We have cooled a cloud of trapped metastable helium atoms to 180 nK, 22 times below the single-photon recoil temperature, with an enhancement in the momentum-space density of 16. We find that the efficiency with which the velocity-selective trap is populated can be augmented by Sisyphus-type frictional forces, while the cooling process is accompanied by the diffusion of atoms to states of very high velocity.
A Very ingenious technique for laser cooling has enabled a group at the Ecole Normale Superieure (ENS) in Paris to reduce the spread in transverse momentum in an atomic beam to the equivalent of 2μK. By using a fundamentally new approach, Alain Aspect and colleagues (Phys. Rev. Lert. 61 826) have overcome the 'recoil limit'. Previously laser radiation has been used to produce, via absorption and re-emission, a frictional force on atoms to damp their motion. With such techniques the minimum energy for the atoms is approximately the recoil energy from emitting a photon; however, experimentally this limit had not been reached.
Dark states were first observed in optical pumping experiments and interpreted as a quenching of fluorescence due to a destructive interference between two absorption amplitudes connecting two ground state sublevels g1 and g2 to an excited sublevel e. The atom can absorb light from g1 and jump to e. Similarly, it can absorb light from g2 and jump to e. But, if it is in a certain linear superposition of g1 and g2, , the two absorption amplitudes from g1 to e and from g2 to e interfere destructively and cancel out. The fluorescence stops. The state from which light cannot be absorbed is called a dark state. This paper will start with a description of the first experiments demonstrating the existence of dark states and of the first theoretical interpretations that have been proposed. Dark states appeared to play an essential role in several new physical effects, such as electromagnetically induced transparency, slow light, stimulated Raman adiabatic passage (STIRAP), which will be briefly described. A special emphasis will be given to the applications of dark states in the field of ultracold atoms and molecules. It will be first shown how the use of velocity dependent dark states allows atoms to be cooled below the so-called recoil limit, corresponding to a velocity dispersion of the cooled atoms smaller than the recoil momentum communicated to the atom by the absorption of a single photon. It turned out that a quantitative interpretation of this subrecoil cooling may be given in terms of anomalous random walks and Lévy statistics. Finally, recent applications of dark states to the production of ultracold molecules will be described: production of ultracold polar molecules by the combination of Feshbach resonances and STIRAP; dark states in the photo-association of ultracold atoms allowing a precise determination of the scattering length describing their collisions.
We report the first observation of subrecoil laser cooling in two dimensions by velocity selective coherent population trapping. We have compressed the momentum distribution of a cloud of trapped metastable helium atoms along two orthogonal axes to $\ensuremath{\delta}p=\frac{\ensuremath{\hbar}k}{4}$, where $\ensuremath{\hbar}k$ is the photon momentum. The corresponding temperature, 16 times below the single-photon recoil temperature, is 250 nK. Atoms are pumped into a nonabsorbing state exhibiting four well-resolved momentum peaks with subrecoil widths. The momentum space density in the peaks is significantly greater than that in the uncooled distribution.
Atoms can now be laser cooled, in one, two and three dimensions, below the limit corresponding to the recoil energy of an atom absorbing or emitting a single photon. Recent works have shown that all subrecoil cooling schemes are based on an anomalous random walk of the atom in momentum space which does not obey the central limit theorem and which introduces a fundamental non–ergodicity in the cooling process. Lévy statistics turn out to be very useful for analysing such a situation since they provide quantitative results for the long time limit of the momentum distribution. In this paper, we briefly review these theoretical works and we give a few references where more details can be found. We also mention a few recent experiments which have been stimulated by these theoretical developments.