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 ...

Recent advances in the precise control of ultracold atomic systems have led to the realisation of Bose Einstein condensates (BECs) and degenerate Fermi gases. An important challenge is to extend this level of control to more complicated molecular systems. One route for producing ultracold molecules is to form them from the atoms in a BEC. For example, a two-photon stimulated Raman transition in a (87)Rb BEC has been used to produce (87)Rb(2) molecules in a single rotational-vibrational state, and ultracold molecules have also been formed through photoassociation of a sodium BEC. Although the coherence properties of such systems have not hitherto been probed, the prospect of creating a superposition of atomic and molecular condensates has initiated much theoretical work. Here we make use of a time-varying magnetic field near a Feshbach resonance to produce coherent coupling between atoms and molecules in a (85)Rb BEC. A mixture of atomic and molecular states is created and probed by sudden changes in the magnetic field, which lead to oscillations in the number of atoms that remain in the condensate. The oscillation frequency, measured over a large range of magnetic fields, is in excellent agreement with the theoretical molecular binding energy, indicating that we have created a quantum superposition of atoms and diatomic molecules two chemically different species.

The point of instability of a Bose-Einstein condensate (BEC) due to attractive interactions was studied. Stable 85Rb BECs were created and then caused to collapse by slowly changing the atom-atom interaction from repulsive to attractive using a Feshbach resonance. At a critical value, an abrupt transition was observed in which atoms were ejected from the condensate. By measuring the onset of this transition as a function of number and attractive interaction strength, we determined the stability condition to be N(absolute value of a) / a(ho) = 0.459+/-0.012+/-0.054, slightly lower than the predicted value of 0.574.