Extreme tunability of interactions in a ⁷Li Bose-Einstein condensate

We use a Feshbach resonance to tune the scattering length a of a Bose-Einstein condensate of 7Li in the |F = 1, m_F = 1> state. Using the spatial extent of the trapped condensate we extract a over a range spanning 7 decades from small attractive interactions to extremely strong repulsive interactions. The shallow zero-crossing in the wing of the Feshbach resonance enables the determination of a as small as 0.01 Bohr radii. In this regime, evidence of the weak anisotropic magnetic dipole interaction is obtained by comparison with different trap geometries.