Entanglement entropy and the large N expansion of two-dimensional Yang-Mills theory

Abstract

Two-dimensional Yang-Mills theory is a useful model of an exactly solvable gauge theory with a string theory dual at large N. We calculate entanglement entropy in the 1/N expansion by mapping the theory to a system of N fermions interacting via a repulsive entropic force. The entropy is a sum of two terms: the "Boltzmann entropy," log dim(R) per point of the entangling surface, which counts the number of distinct microstates, and the "Shannon entropy," - Sigma p(R) log p(R), which captures fluctuations of the macroscopic state. We find that the entropy scales as N-2 in the large N limit, and that at this order only the Boltzmann entropy contributes. We further show that the Shannon entropy scales linearly with N, and confirm this behaviour with numerical simulations. While the term of order N is surprising from the point of view of the string dual - in which only even powers of N appear in the partition function - we trace it to a breakdown of large N counting caused by the replica trick. This mechanism could lead to corrections to holographic entanglement entropy larger than expected from semiclassical field theory.