Observation of Dipole-Dipole Interaction in a Degenerate Quantum Gas

We have investigated the expansion of a Bose-Einstein condensate of strongly magnetic chromium atoms. The long-range and anisotropic magnetic dipole-dipole interaction leads to an anisotropic deformation of the expanding chromium condensate which depends on the orientation of the atomic dipole moments. Our measurements are consistent with the theory of dipolar quantum gases and show that a chromium condensate is an excellent model system to study dipolar interactions in such gases.

Figure 1

Aspect ratio of a freely expanding Cr-BEC for two different directions of magnetization induced by a homogeneous magnetic field ($\overrightarrow{B}$). Blue: experimental data (circles) and theoretical prediction without adjustable parameter (solid line) for transversal magnetization (atomic dipoles $\overrightarrow{\mu }$ aligned orthogonal to the weak trap axis). Red: data (diamonds) and theory curve (solid line) for longitudinal magnetization ($\overrightarrow{\mu }$ parallel to the weak trap axis). Blue upward and red downward triangles are results of 31 measurements taken 10 ms after release for transversal and longitudinal magnetization, respectively. Dashed line: theory curve without dipole-dipole interaction. The inset (upper left corner) sketches the in-trap BEC. The BEC images at the right axis illustrate the condensate shape for some aspect ratios.

Figure 2

Top: sketch of the atomic density distribution $n(\overrightarrow{r})$ of a nondipolar BEC in the Thomas-Fermi regime within a spherically symmetric harmonic trap ($x,z$-plane cross section through the center of trap). Bottom: asymmetric dipole-dipole interaction potential ${\Phi }_{\mathrm{dd}}(\overrightarrow{r})$ (cross section like in top) for a dipolar BEC with density distribution $n(\overrightarrow{r})$. Within the atomic cloud, ${\Phi }_{\mathrm{dd}}(\overrightarrow{r})$ has the form of a saddle with negative curvature along the direction of magnetization (sketched by the magnets with dipole moment $\overrightarrow{\mu }$ and the $\overrightarrow{B}$ vector) and positive curvature orthogonal to it.