  We revisit from a quantum-information perspective a classic problem of
polaron theory in one dimension. In the context of the Holstein model we show
that a simple analysis of quantum entanglement between excitonic and phononic
degrees of freedom allows one to effectively characterize both the small and
large polaron regimes as well as the crossover in between. The small (large)
polaron regime corresponds to a high (low) degree of bipartite quantum
entanglement between the exciton and the phonon cloud that clothes the exciton.
Moreover, the self-trapping transition is clearly displayed by a sharp drop of
exciton-phonon entanglement.
