Collective enhancement and suppression in Bose-Einstein condensates

The coherent and collective nature of Bose-Einstein condensate can enhance or suppress physical processes. Bosonic stimulation enhances scattering in already occupied states which leads to atom amplification, and the suppression of dissipation leads to superfluidity. In this paper, we review several experiments where suppression and enhancement have been observed and discuss the common roots of and differences between these phenomena. When a gas of bosonic atoms is cooled below the transition temperature of BoseEinstein 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 . The theoretical framework connects the observed equilibrium and dynamic properties to the presence of longrange order and low-lying collective excitations [7]. Many special properties of Bose condensates involve the suppression or enhancement of physical processes. Our recent experiments include the suppression and enhancement of elastic collisions of impurity atoms [8], the suppression of dissipation due to superfluidity [9,10], and the suppression [11] and enhancement of light scattering [12]. Bosonically enhanced Rayleigh scattering was used to amplify either atoms [13] or light [14] in a condensate dressed by laser light. We review these experiments and discuss the properties of the Bose condensate which lead to enhancement and suppression. I 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 which will be useful to see the similarities and differences between the different processes. When a condensate scatters a photon or material particle, the scattering is described by the Hamiltonian