Stabilization of a purely dipolar quantum gas against collapse

Although the phenomenon of Bose-Einstein condensation(1) is a purely statistical effect that also appears in an ideal gas, the physics of Bose-Einstein condensates (BECs) of dilute gases is considerably enriched by the presence of interactions among the atoms. In usual experiments with BECs, the only relevant interaction is the isotropic and short-range contact interaction, which is described by a single parameter, the scattering length a. In contrast, the dipole-dipole interaction between particles possessing an electric or magnetic dipole moment is of long-range character and anisotropic, which gives rise to new phenomena(2,3). Most prominently, the stability of a dipolar BEC depends not only on the value of the scattering length, a, but also strongly on the geometry of the external trapping potential(4-7). Here, we report on the experimental investigation of the stability of a dipolar BEC of Cr-52 as a function of the scattering length and the trap aspect ratio. We find good agreement with a universal stability threshold arising from a simple theoretical model. Using a pancake-shaped trap with the dipoles oriented along the short axis of the trap, we are able to tune the scattering length to zero, stabilizing a purely dipolar quantum gas.