Collisional properties of ultracold Chromium: Towards a purely dipolar quantum gas

Due to its large magnetic moment of 6 mu_B, atomic chromium is the only Bose-Einstein condensed species so far to show relatively large magnetic dipole-dipole interactions (MDDI) besides the usual s-wave contact interaction. The long-range (1/r³) and anisotropic character of the MDDI gives birth to unique properties for the Cr Bose-Einstein condensate obtained recently in our group. In particular, it leads to an anisotropic deformation of the expanding condensate which depends on the orientation of the atomic dipole moments (Giovanazzi et al., 2006). In these measurements the dominant interaction is the contact one. We used expansion dynamics measurements to quantify the ratio between dipolar and contact interactions to be epsiv_dd = 0.159 plusmn 0.034 . This in turn provides a new experimental determination of the chromium scattering length: a = (96 plusmn 20) a₀ with an accuracy which is not limited by errors in the measurement of the atom number.