Exact evaluation of density matrix elements for the Heisenberg chain

Abstract

We have obtained all the density matrix elements on six lattice sites for the spin-1/2Heisenberg chain via the algebraic method based on the quantum Knizhnik–Zamolodchikovequations. Several interesting correlation functions, such as chiral correlation functions,dimer–dimer correlation functions, etc, have been analytically evaluated. Furthermore, wehave calculated all the eigenvalues of the density matrix and analyse the eigenvaluedistribution. As a result, the exact von Neumann entropy for the reduced density matrix onsix lattice sites has been obtained.