Comparing contact and dipolar interaction in a Bose-Einstein condensate

We have measured the relative strength ${ϵ}_{d}d$ of the magnetic dipole-dipole interaction compared to the contact interaction in a chromium Bose-Einstein condensate. We analyze the asymptotic velocities of expansion of a dipolar chromium BEC with different orientations of the atomic magnetic dipole moments. By comparing them with numerical solutions of the hydrodynamic equations for dipolar condensates, we are able to determine $\epsilon_dd = 0.159\pm0.034$ with high accuracy. Since the absolute strength of the dipole-dipole interaction is known exactly, the relative strength of the dipoledipole interaction can be used to determine the s-wave scattering length $a=5.08\pm 1.06\cdot {10}^{-}9m=96\pm 20a0$ of 52Cr. This is fully consistent with our previous measurements on the basis of Feshbach resonances.