Sufficiently strong modulation of the interaction strength in a Bose-Einstein condensate induces inelastic atom-atom scattering and causes collective emission of matter-wave jets from the condensate.
Scattering is used to probe matter and its interactions in all areas of physics. In ultracold atomic gases, control over pairwise interactions enables us to investigate scattering in quantum many-body systems1. Previous experiments on colliding Bose-Einstein condensates have revealed matter-wave interference2,3, haloes of scattered atoms4,5, four-wave mixing6,7 and correlations between counter-propagating pairs8,9,10. However, a regime with strong stimulation of spontaneous collisions11,12,13,14,15,16,17,18,19,20 analogous to superradiance21,22,23 has proved elusive. In this regime, the collisions rapidly produce highly correlated states with macroscopic population. Here we find that runaway stimulated collisions in Bose-Einstein condensates with periodically modulated interaction strength cause the collective emission of matter-wave jets that resemble fireworks. Jets appear only above a threshold modulation amplitude and their correlations are invariant even when the number of ejected atoms grows exponentially. Hence, we show that the structures and atom occupancies of the jets stem from the quantum fluctuations of the condensate. Our findings demonstrate the conditions required for runaway stimulated collisions and reveal the quantum nature of matter-wave emission.