Entanglement in Quantum Spin Chains, Symmetry Classes of Random Matrices, and Conformal Field Theory

We compute the entropy of entanglement between the first N spins and the rest of the system in the ground states of a general class of quantum spin chains. We show that under certain conditions the entropy can be expressed in terms of averages over ensembles of random matrices. These averages can be evaluated, allowing us to prove that at critical points the entropy grows like κlog₂N+κ as N→∞, where κ and κ are determined explicitly. In an important class of systems, κ is equal to one-third of the central charge of an associated Virasoro algebra. Our expression for κ therefore provides an explicit formula for the central charge.