Strong dipolar effects in a quantum ferrofluid

Symmetry-breaking interactions have a crucial role in many areasof physics, ranging from classical ferrofluids to superfluid 3He andd-wave superconductivity. For superfluid quantum gases, a varietyof new physical phenomena arising from the symmetry-breakinginteraction between electric or magnetic dipoles are expected1.Novel quantum phases in optical lattices, such as chequerboardor supersolid phases, are predicted for dipolar bosons2,3. Dipolarinteractions can also enrich considerably the physics of quantumgases with internal degrees of freedom4-6. Arrays of dipolar particlescould be used for efficient quantum information processing7.Here we report the realization of a chromium Bose-Einstein condensatewith strong dipolar interactions. By using a Feshbachresonance, we reduce the usual isotropic contact interaction, suchthat the anisotropic magnetic dipole-dipole interaction between52Cr atoms becomes comparable in strength. This induces achange of the aspect ratio of the atom cloud; for strong dipolarinteractions, the inversion of ellipticity during expansion (theusual 'smoking gun' evidence for a Bose-Einstein condensate)can be suppressed. These effects are accounted for by taking intoaccount the dipolar interaction in the superfluid hydrodynamicequations governing the dynamics of the gas, in the same way asclassical ferrofluids can be described by including dipolar terms inthe classical hydrodynamic equations. Our results are a first stepin the exploration of the unique properties of quantum ferrofluids.