  We investigate the nature of quantum entanglement in long-range orders and
spontaneous symmetry breaking. It is shown that diminishing of entanglement
between the condensate mode and the rest of the system underlies off-diagonal
long-range order, which is the hallmark of superconductivity and Bose-Einstein
condensation. It is also revealed that disentanglement underlies various cases
of long-range order and spontaneous symmetry breaking. In the course of the
discussion, we also present some ideas on characterizing entanglement in
many-body systems. Especially, it is shown how the connected correlation
functions can be used in characterizing entanglements in a pure state.
