M. R. Andrews

Observation of Feshbach resonances in a Bose–Einstein condensate

It has long been predicted that the scattering of ultracold atoms can be altered significantly through a so-called ‘Feshbach resonance’. Two such resonances have now been observed in optically trapped Bose–Einstein condensates of sodium atoms by varying an external magnetic field. They gave rise to enhanced inelastic processes and a dispersive variation of the scattering length by a factor of over ten. These resonances open new possibilities for the study and manipulation of Bose–Einstein condensates.

Optical Confinement of a Bose-Einstein Condensate

Bose-Einstein condensates of sodium atoms have been confined in an optical dipole trap using a single focused infrared laser beam. This eliminates the restrictions of magnetic traps for further studies of atom lasers and Bose-Einstein condensates. More than five million condensed atoms were transferred into the optical trap. Densities of up to

Direct, Nondestructive Observation of a Bose Condensate

3 \times 10^{15} cm^{-3}

Strongly Enhanced Inelastic Collisions in a Bose-Einstein Condensate near Feshbach Resonances

of Bose condensed atoms were obtained, allowing for a measurement of the three-body decay rate constant for sodium condensates as

OPTICALLY CONFINED BOSE-EINSTEIN CONDENSATES

K_3 = (1.1 \pm 0.3) \times 10^{-30} cm^6 s^{-1}

BOSE-EINSTEIN CONDENSATION OF ULTRACOLD ATOMIC GASES

. At lower densities, the observed 1/e lifetime was more than 10 sec. Simultaneous confinement of Bose-Einstein condensates in several hyperfine states was demonstrated.

Studies of Bose-Einstein Condensates

The spatial observation of a Bose condensate is reported. Dispersive light scattering was used to observe the separation between the condensed and normal components of the Bose gas inside a magnetic trap. This technique is nondestructive, and about a hundred images of the same condensate can be taken. The width of the angular distribution of scattered light increased suddenly at the phase transition.

Bose-Einstein condensates—a new form of quantum matter

The properties of Bose-Einstein condensed gases can be strongly altered by tuning the external magnetic field near a Feshbach resonance. Feshbach resonances affect elastic collisions and lead to the observed modification of the scattering length. However, as we report here, the observed rate of inelastic collisions was strongly enhanced in a sodium Bose-Einstein condensate when the scattering length was tuned to both larger or smaller values than the off-resonant value. These strong losses impose severe limitations for using Feshbach resonances to tune the properties of Bose-Einstein condensates. [S0031-9007(99)08767-0] Most of the properties of Bose-Einstein condensates in dilute alkali gases are dominated by two-body collisions, which can be characterized by the s-wave scattering length a. The sign and the absolute value of the scattering length determine, e.g., stability, internal energy, formation rate, size, and collective excitations of a condensate. Near a Feshbach resonance the scattering length varies dispersively [1,2] covering the whole range of positive and negative values. Thus it should be possible to study strongly interacting, weakly or noninteracting, or collapsing condensates [3], all with the same alkali species and experimental setup. A Feshbach resonance occurs when the energy of a molecular (quasi-) bound state is tuned to the energy of two colliding atoms by applying an external magnetic field. Such resonances have been observed in a BoseEinstein condensate of Na sF › 1, mF › 11) atoms at 853 and 907 G [4,5], and in two experiments with cold clouds of 85 Rb (F › 2, mF › 22) atoms at 164 G [6]. In the sodium experiment, the scattering length a was observed to vary dispersively as a function of the magnetic field B, in agreement with the theoretical prediction [2]:

Bose-Einstein Condensation in a Gas of Sodium Atoms

With an optical dipole trap it is possible to confine Bose–Einstein condensates in different hyperfine states and in arbitrary magnetic bias fields, thus overcoming two major limitations of magnetic traps. In this review paper we characterize the properties of such a dipole trap and we summarize experiments which made use of the new experimental possibilities, including the reversible formation of a Bose–Einstein condensate, the observation of Feshbach resonances in sodium and the ground state properties of spinor Bose-Einstein condensates. Finally, we present some new results on the shape of magnetically trapped and ballistically expanding condensates.

Bose-Einstein-condensation and prospects for precision measurements

Bose–Einstein condensation in a dilute gas of sodium atoms has been observed. The atoms were trapped in an optically plugged magnetic trap or in a cloverleaf magnetic trap. Rf induced evaporative cooling increased the phase-space density by six orders of magnitude. We summarize the different techniques used, and discuss recent studies of properties of Bose condensates with an outlook on future developments.