  We analytically study entanglement generation through an open quantum dot
system described by the two-lead Anderson model. We exactly obtain the
transition rate between the non-entangled incident state in one lead and the
outgoing spin-singlet state in the other lead. In the cotunneling process, only
the spin-singlet state can transmit. To discuss such an entanglement property
in the open quantum system, we construct the exact two-electron scattering
state of the Anderson model. It is striking that the scattering state contains
spin-singlet bound states induced by the Coulomb interaction. The bound state
describes the scattering process in which the set of momenta is not conserved
and hence it is not in the form of a Bethe eigenstate.
