  The prediction of the response of a closed system to external perturbations
is one of the central problems in quantum mechanics, and in this respect, the
local density of states (LDOS) provides an in- depth description of such a
response. The LDOS is the distribution of the overlaps squared connecting the
set of eigenfunctions with the perturbed one. Here, we show that in the case of
closed systems with classically chaotic dynamics, the LDOS is a Breit-Wigner
distribution under very general perturbations of arbitrary high intensity.
Consequently, we derive a semiclassical expression for the width of the LDOS
which is shown to be very accurate for paradigmatic systems of quantum chaos.
This Letter demonstrates the universal response of quantum systems with
classically chaotic dynamics.
