Pumping atoms into a Bose-Einstein condensate in the boson-accumulation regime.

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We consider the behavior of an atomic Bose-Einstein condensate in the presence of an atom in an (internal) excited electronic level. We analyze the boson-accumulation regime, defined by the relation ${\mathit{N}}_{0}$\ensuremath{\gg}a,N-${\mathit{N}}_{0}$, where N is the total number of atoms in the ground electronic state, ${\mathit{N}}_{0}$ is the number of atoms in the condensate, and a is the number of levels to which the excited atom can decay. In this regime, quantum statistical effects related to the boson nature of the atoms are predominant in the process of spontaneous emission. Simple arguments suggest that the proportion of atoms in the condensate decreases after the spontaneous decay. A more elaborated model based on a master equation description shows that, in general, these arguments may lead to erroneous conclusions. Under certain conditions it predicts that the proportion of atoms in the condensate increases. We give an interpretation of this phenomenon in terms of quantum interferences between processes that include reabsorption of the emitted photons, and dipole-dipole interactions between the atoms. The physical effect considered here may help to pump atoms into a condensate and to compensate for atomic losses in the atomic trap. \textcopyright{} 1996 The American Physical Society.