  We develop in this letter an analytical approach using form- factors to
compute time dependent correlations in integrable quantum impurity problems. As
an example, we obtain for the first time the frequency dependent conductivity
$G(\omega)$ for the tunneling between the edges in the $\nu=1/3$ fractional
quantum Hall effect, and the spectrum $S(w)$ of the spin-spin correlation in
the anisotropic Kondo model and equivalently in the double well system of
dissipative quantum mechanics, both at vanishing temperature.
