How do interacting bosons scatter light?

Bosonic stimulation of physical processes is a primary manifestation of quantum statistics, relevant for the operation of lasers and the formation of Bose–Einstein condensates. In the textbook picture, scattering of identical bosonic atoms between different (spatially delocalized) momentum states is enhanced by their tendency to join already occupied states. However, this enhancement is fundamentally caused by local fluctuations, and the textbook formulation in terms of global state occupations is simply applicable only to non-interacting particles. Here, by studying the scattering between off-resonant light and atoms in a quasi-homogeneous Bose gas, we show that atomic interactions can, without significantly affecting the global momentum-space occupations, dramatically affect the bosonic enhancement of scattering – due to beyond-mean-field effects, even weak repulsive interactions can almost completely suppress it, while weak attractive interactions enhance it. Moreover, if the interactions are rapidly tuned, the atomic correlations revealed by light scattering change on a timescale that is orders of magnitude shorter than the collisional timescale for changing the momentum-space occupations. Owing to this extreme sensitivity to dynamical beyond-mean-field effects, off-resonant light scattering has promise as a simple and powerful new probe of many-body physics in ultracold atomic gases.