Direct observation of growth and collapse of a Bose–Einstein condensate with attractive interactions

Quantum theory predicts that Bose–Einstein condensation of a spatially homogeneous gas with attractive interactions is precluded by a conventional phase transition into either a liquid or solid1. When confined to a trap, however, such a condensate can form2, provided that its occupation number does not exceed a limiting value3,4. The stability limit is determined by a balance between the self-attractive forces and a repulsion that arises from position–momentum uncertainty under conditions of spatial confinement. Near the stability limit, self-attraction can overwhelm the repulsion, causing the condensate to collapse5,6,7,8. Growth of the condensate is therefore punctuated by intermittent collapses9,10 that are triggered by either macroscopic quantum tunnelling or thermal fluctuation. Previous observations of growth and collapse dynamics have been hampered by the stochastic nature of these mechanisms. Here we report direct observations of the growth and subsequent collapse of a 7Li condensate with attractive interactions, using phase-contrast imaging. The success of the measurement lies in our ability to reduce the stochasticity in the dynamics by controlling the initial number of condensate atoms using a two-photon transition to a diatomic molecular state.