Wave functions, relative phase and interference for atomic Bose-Einstein condensates

In this article, we present a tutorial discussion of the coherence properti es of Bose-Einstein condensates. We use a formalism which is similar to the one used in quantum optics. We describe within the variational approximati on the question of the relative phase of two condensates. To evaluate the s tructure factors of a condensate, we briefly review the Bogolubov approach and describe light scattering off a condensate using the linear response fo rmalism. Finally, we study the effect of atomic interactions on the condens ate's dynamics. (C) 2001 Academie des sciences/Editions scientifiques ct me dicales Elsevier SAS.