  We study the entanglement of an impurity at one end of a spin chain with a
block of spins using negativity as a true measure of entanglement to
characterize the unique features of the gapless Kondo regime in the spin chain
Kondo model. For this spin chain in the Kondo regime we determine- with a true
entanglement measure- the spatial extent of the Kondo screening cloud, we
propose an ansatz for its ground state and demonstrate that the impurity spin
is indeed maximally entangled with the cloud. To better evidence the
peculiarities of the Kondo regime, we carry a parallel analysis of the
entanglement properties of the Kondo spin chain model in the gapped dimerised
regime. Our study shows how a genuine entanglement measure stemming from
quantum information theory can fully characterize also non perturbative regimes
accessible to certain condensed matter systems.
