CONDENSATS DE BOSE–EINSTEIN ET LASERS À ATOMES BOSE–EINSTEIN CONDENSATES AND ATOM LASERS Collective enhancement and suppression in Bose–Einstein condensates

The coherent and collective nature of a Bose–Einstein condensate can enhance or suppress physical processes. Bosonic stimulation enhances scattering in already occupied states which leads to matter wave amplification, and the suppression of dissipation leads to superfluidity. In this article we present several experiments where enhancement and suppression have been observed and discuss the common roots of and differences between these phenomena.  2001 Académie des sciences/Éditions scientifiques et médicales Elsevier SAS Bose–Einstein condensation / superfluidity / structure factor / superradiance / amplification of light / matter wave amplification Effets collectifs d’exaltation ou de réduction dans les condensats de Bose–Einstein Résumé. La nature cohérente et collective d’un condensat de Bose-Einstein peut conduire à l’exaltation ou à la réduction d’effets physiques. La stimulation bosonique augmente la diffusion vers des états déjà occupés, ce qui conduit à l’amplification des ondes de matière, tandis que la réduction de la dissipation conduit à la superfluidité. Dans cet article, nous présentons plusieurs expériences où augmentation et réduction ont été observées, et nous discutons les points communs et les différences entre ces phénomènes.  2001 Académie des sciences/Éditions scientifiques et médicales Elsevier SAS condensation Bose–Einstein / superfluidité / facteur de structure / super radiance / amplification de lumiére / amplification matiére ondes When a gas of bosonic atoms is cooled below the transition temperature of Bose–Einstein condensation, it profoundly changes its properties. The appearance of a macroscopically occupied quantum state leads to a variety of new phenomena which set quantum fluids apart from all other substances. Fritz London even called them the fourth state of matter [1]. Many of the key concepts in quantum fluids were derived from studying the weakly interacting Bose gas, for which rigorous theoretical treatments were possible [2,3]. In 1995, with the discovery of BEC in a dilute gas of alkali atoms [4–6], it became possible to study such a system experimentally. Since then, the interplay between theory and experiment has been remarkable [7]. Note présentée par Guy LAVAL. S1296-2147(01)01180-5/FLA  2001 Académie des sciences/Éditions scientifiques et médicales Elsevier SAS. Tous droits réservés 339 W. Ketterle, S. Inouye BOSE–EINSTEIN CONDENSATES AND ATOM LASERS This paper is the fourth major review paper of our group which describes the new techniques and the physics of Bose–Einstein condensation [8–10]. These review papers together give a comprehensive overview of the topics to which our group has contributed. Each contribution is self-contained, but we avoided major overlap with previous review papers. The topic of these notes is enhancement and suppression of physical processes in a Bose–Einstein condensate. Many phenomena in Bose condensates involve such enhancement or suppression. Our recent experiments include the enhancement and suppression of elastic collisions of impurity atoms [11], the suppression of dissipation due to superfluidity [12,13], and the enhancement [14] and suppression [15] of light scattering. The common discussion of these phenomena leads to a better understanding of the underlying principles. We draw analogies between light scattering and particle scattering, between microscopic and macroscopic superfluidity. We show that a condensate responds very differently to two different ways of momentum transfer, light scattering and spontaneous emission [16]. We discuss light scattering in both the linear and nonlinear regime where bosonically enhanced Rayleigh scattering led to the amplification of either atoms [17] or light [18] and work out the relationship between these two processes. Finally, the section on matter wave amplification of fermions discusses the relevance of symmetry and long coherence time and its relation to quantum statistics. 1. Scattering of light and massive particles Before we discuss light scattering and collisions in a BEC, we want to derive some simple general expressions based on Fermi’s golden rule. This will help to see the similarities and differences between the various processes. When a condensate scatters a photon or material particle, the scattering is described by the Hamiltonian: