  We develop a numerical method to compute the negativity, an entanglement
measure for mixed states, between the impurity and the bath in quantum impurity
systems at finite temperature. We construct a thermal density matrix by using
the numerical renormalization group (NRG), and evaluate the negativity by
implementing the NRG approximation that reduces computational cost
exponentially. We apply the method to the single-impurity Kondo model and the
single-impurity Anderson model. In the Kondo model, the negativity exhibits a
power-law scaling at temperature much lower than the Kondo temperature and a
sudden death at high temperature. In the Anderson model, the charge fluctuation
of the impurity contribute to the negativity even at zero temperature when the
on-site Coulomb repulsion of the impurity is finite, while at low temperature
the negativity between the impurity spin and the bath exhibits the same
power-law scaling behavior as in the Kondo model.
