科研之友
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摘要
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 g(1) and g(2) to an excited sublevel e. The atom can absorb light from g(1) and jump to e. Similarly, it can absorb light from g(2) and jump to e. But, if it is in a certain linear superposition of g(1) and g(2), c(1)g(1) + c(2)g(2), the two absorption amplitudes from g(1) to e and from g(2) to e interfere destructively and cancel out. The fluorescence stops. The state c(1)g(1) + c(2)g(2) 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 Levy 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.
- 出版日期2015-8
