Dark region observed in fluorescence under the condition of a quadrupole magnetic field - art. no. 023406

We report an experimental observation of a dark region, around the zero-fie ld point of a spheric quadrupole magnetostatic field, in spatial fluorescen ce emitted by dilute vapor atoms, which were excited by a circularly polari zed traveling-wave laser tuned to a F-->F+1 transition of Rb-87. By ruling out some possible sources, we show that the fluorescence decrease is associ ated with the three-dimensional nature of the magnetic field; it is a conse quence of a precession of magnetic dipole moments induced by the radial mag netic field, which significantly cancels the orientation caused by optical pumping near the point of level crossing in the presence of the axial magne tic held. In an alternative way, the phenomenon can be understood as result ing from a destructive quantum interference, which is related to a coherent double-quantum coupling of atomic levels with two fields; one is the laser field, the other is the magnetic field. Analytic expressions for the half- width and contrast of the dark region have been obtained with a simplified model in the limit of low laser power. A qualitative agreement between the theory and experiment is found. The relevance of our study to laser cooling and trapping in a widely used magneto-optical trap based on such a quadrup ole magnetic field is discussed. Finally, some applications of the phenomen on are also presented.