Replica Instantons from Axion-like Coupling

Abstract: We find a phenomenon in a non-gravitational gauge theory analogous to the replica wormhole in a quantum gravity theory. We consider a reservoir of a scalar field coupled with a gauge theory contained in a region with a boundary by an axion-like coupling. When the replica trick is used to compute the entanglement entropy for a subregion in the reservoir, a tuple of instantons distributed across the replica sheets gives a non-perturbative contribution. As an explicit and solvable example, we consider a discrete scalar field coupled to a 2d pure gauge theory and observe how the replica instantons reproduce the entropy directly calculated from the reduced density matrix. In addition, we notice that the entanglement entropy can detect the confinement of a 2d gauge theory.