Bosonic stimulation of atom–light scattering in an ultracold gas

For bosons, the transition rate into an already occupied quantum state is enhanced by its occupation number: the effect of bosonic stimulation. Bosonic stimulation of light scattering was predicted more than 30 years ago but has proven elusive to direct observation. Here we investigate this effect in an ultracold gas of bosons. We show that the bosonic enhancement factor for a harmonically trapped gas is bounded by a universal constant above the phase transition to a Bose–Einstein condensate and depends linearly on the condensate fraction just below the phase transition. We observe bosonic enhanced light scattering both above and below the phase transition, and we show how interactions can alter the bosonic stimulation and optical properties of the gas. Lastly, we demonstrate that, for a multi-level system prepared in a single internal state, the bosonic enhancement is reduced because it occurs only for Rayleigh scattering but not for Raman scattering. In bosonic systems, the presence of particles in a given quantum level can enhance the transition rates into that state, an effect known as bosonic stimulation. Bosonic enhancement of light scattering has now been observed in an ultracold Bose gas.