Collective enhancement and suppression in Bose-Einstein condensates

The coherent and collective nature of a Bose-Einstein condensate can enhanc e or suppress physical processes. Bosonic stimulation enhances scattering i n already occupied states which leads to matter wave amplification, and the suppression of dissipation leads to superfluidity. In this article we pres ent several experiments where enhancement and suppression have been observe d and discuss the common roots of and differences between these phenomena. (C) 2001 Academie des sciences/Editions scientifiques et medicales Elsevier SAS.