We explored the dynamics of how a Bose-Einstein condensate collapses and subsequently explodes when the balance of forces governing the size and shape of the condensate is suddenly altered. A condensate's equilibrium size and shape is strongly affected by the inter-atomic interactions. Our ability to induce a collapse by switching the interactions from repulsive to attractive by tuning an externally-applied magnetic field yields a wealth of detailed information on the violent collapse process. We observe anisotropic atom bursts that explode from the condensate, atoms leaving the condensate in undetected forms, spikes appearing in the condensate wave function, and oscillating remnant condensates that survive the collapse. These all have curious dependencies on time, the strength of the interaction, and the number of condensate atoms. Although ours would seem to be a simple well-characterized system, our measurements reveal many interesting phenomena that challenge theoretical models.Although the density of the atoms in an atomic BoseEinstein condensate (BEC) is typically five orders of magnitude lower than the density of air, the inter-atomic interactions greatly affect a wide variety of BEC properties. These include static properties like the BEC size and shape and the condensate stability, and dynamic properties like the collective excitation spectrum and soliton and vortex behavior. Since all of these properties are sensitive to the inter-atomic interactions, they can be quite dramatically affected by tuning the interaction strength and sign.The vast majority of BEC physics is well described by mean-field theory 1 , in which the strength of the interactions depends on the atom density and on one additional parameter called the s-wave scattering length a. a is determined by the atomic species. When a > 0, the interactions are repulsive. In contrast, when a < 0 the interactions are attractive and a BEC tends to contract to minimize its overall energy. In a harmonic trap, the contraction competes with the kinetic zero-point energy, which tends to spread out the condensate. For a strong enough attractive interaction, there is not enough kinetic energy to stabilize the BEC and it is expected to implode. A BEC can avoid implosion only as long as the number of atoms N 0 is less than a critical value given by 2where dimensionless constant k is called the stability coefficient. The precise value of k depends on the aspect ratio of the magnetic trap 3 . a ho is the harmonic oscillator length, which sets the size of the condensate in the ideal-gas (a = 0) limit. Under most circumstances, a is insensitive to external fields. This is different in the vicinity of a so-called Feshbach resonance, where a can be tuned over a huge range by adjusting the externally applied magnetic field 4,5 . This has been demonstrated in recent years with cold 85 Rb and Cs atoms 6,7,8 , and with Na and 85 Rb BoseEinstein condensates 9,10 . For 85 Rb atoms, a is usually negative, but a Feshbach resonance at ∼155 G allows us to tune a by orders ...

Bose-Einstein condensation has been achieved in a magnetically trapped sample of 85 Rb atoms. Long-lived condensates of up to 10 4 atoms have been produced by using a magnetic-field-induced Feshbach resonance to reverse the sign of the scattering length. This system provides many unique opportunities for the study of condensate physics. The variation of the scattering length near the resonance has been used to magnetically tune the condensate selfinteraction energy over a very wide range. This range extended from very strong repulsive to large attractive self-interactions. When the interactions were switched from repulsive to attractive, the condensate shrank to below our resolution limit, and after ∼ 5 ms emitted a burst of high-energy atoms.Typeset using REVT E X 1

We observe bright matter-wave solitons form during the collapse of (85)Rb condensates in a three-dimensional (3D) magnetic trap. The collapse is induced by using a Feshbach resonance to suddenly switch the atomic interactions from repulsive to attractive. Remnant condensates containing several times the critical number of atoms for the onset of instability are observed to survive the collapse. Under these conditions a highly robust configuration of 3D solitons forms such that each soliton satisfies the condition for stability and neighboring solitons exhibit repulsive interactions.