A Bose-Einstein condensate was produced in a vapor of rubidium-87 atoms that was confined by magnetic fields and evaporatively cooled. The condensate fraction first appeared near a temperature of 170 nanokelvin and a number density of 2.5 x 1012 per cubic centimeter and could be preserved for more than 15 seconds. Three primary signatures of Bose-Einstein condensation were seen. (i) On top of a broad thermal velocity distribution, a narrow peak appeared that was centered at zero velocity. (ii) The fraction of the atoms that were in this low-velocity peak increased abruptly as the sample temperature was lowered. (iii) The Fig. 1. The optical components and magnetic coils are all located outside the ultrahigh-vacuum glass cell, which allows for easy access and modification. Rubidium atoms from the background vapor were optically precooled and trapped, loaded into a magnetic trap, then further cooled by evaporation. The TOP (time orbiting potential) magnetic trap (13) we used is a superposition of a large spherical quadrupole field and a small uniform transverse field that rotates at 7.5 kHz. This arrangement results in an effective average potential that is an axially symmetric, three-dimensional (3D) harmonic potential providing tight and stable confinement during evaporation. The evaporative cooling works by selectively re-

We have created vortices in two-component Bose-Einstein condensates. The vortex state was created through a coherent process involving the spatial and temporal control of interconversion between the two components. Using an interference technique, we map the phase of the vortex state to confirm that it possesses angular momentum. We can create vortices in either of the two components and have observed differences in the dynamics and stability.

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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 ...