  We show that quasi-Nambu-Goldstone (NG) modes, which play prominent roles in
high energy physics but have been elusive experimentally, can be realized with
atomic Bose-Einstein condensates. The quasi-NG modes emerge when the symmetry
of a ground state is larger than that of the Hamiltonian. When they appear, the
conventional vacuum manifold should be enlarged. Consequently topological
defects that are stable within the conventional vacuum manifold become unstable
and decay by emitting the quasi-NG modes. Contrary to conventional wisdom,
however, we show that the topological defects are stabilized by quantum
fluctuations that make the quasi-NG modes massive, thereby suppressing their
emission.
