Observation of bosonic stimulation in light scattering

For bosons, the transition rate into an already occupied quantum state is enhanced by its occupation number: the effect of bosonic stimulation. Bosonic enhancement of light scattering has been predicted more than 30 years ago but not observed before. Here we theoretically investigate and experimentally demonstrate this effect in an ultracold Bose gas. We show that the bosonic enhancement factor for a harmonically trapped gas is bounded by a universal constant ζ(2)/ζ(3) above the phase transition to a Bose-Einstein condensate (BEC), and depends linearly on the BEC fraction just below the phase transition. Bosonically enhanced light scattering is observed and characterized above and below the phase transition, and the correction due to interaction is discussed. For a multi-level system, bosonic enhancement is reduced because bosonic stimulation occurs only for Rayleigh scattering, but not for Raman scattering.