  Rydberg blockade sphere persists an intriguing picture by which a number of
collective many-body effects caused by the strong Rydberg-Rydberg interactions
can be clearly understood and profoundly investigated. In the present work, we
develop a new definition for the effective two-atom blockade radius and show
that the original spherically shaped blockade surface would be deformed when
the real number of atoms increases from two to three. This deformation of
blockade sphere reveals spatially anisotropic and shrunken properties which
strongly depend on the interatomic distance. In addition, we also study the
optimal conditions for the Rydberg antiblockade effect and make predictions for
improving the antiblockade efficiency in few-atom systems.
