Entanglement and fluctuations in the Majumdar-Ghosh model ground state


The matrix product state representation of the ground state of the Majumdar–Ghosh Model (MGM) is used to obtain the reduced density matrix of a block of x spins in a periodic MGM chain. This calculation leads to exact expressions for the entanglement entropy and subsystem fluctuations as a function of block length. We find that the entanglement entropy saturates exponentially to ln 3, even as the subsystem Hilbert space is five-dimensional.