Collective oscillations of dipolar Bose-Einstein condensates and accurate comparison between contact and dipolar interactions

We propose a scheme for the measurement of the s-wave scattering length a of an atom or molecule with significant dipole-dipole interaction with an accuracy at the percent level. The frequencies of the collective oscillations of a Bose-Einstein condensate are shifted by the magnetic dipole interaction. The shift is polarization dependent and proportional to the ratio epsilon(dd) of dipolar and s-wave coupling constants. Measuring the differences in the frequencies for different polarization we can extract the value of epsilon(dd) and thus measure a. We calculate the frequency shifts for a large variety of nonaxisymmetric harmonic traps in the Thomas-Fermi limit and find optimal trapping geometries to maximize the shifts.