  We report the observation of the controlled expansion of a two-dimensional
quantum gas confined onto a curved shell-shaped surface. We start from the
ellipsoidal geometry of a dressed quadrupole trap and introduce a novel gravity
compensation mechanism enabling to explore the full ellipsoid. The zero-point
energy of the transverse confinement manifests itself by the spontaneous
emergence of an annular shape in the atomic distribution. The experimental
results are compared with the solution of the three-dimensional
Gross-Pitaevskii equation and with a two-dimensional semi-analytical model.
This work evidences how a hidden dimension can affect dramatically the embedded
low-dimensional system by inducing a change of topology.
