Abstract: | In the Unruh effect, the Minkowski vacuum of a quantum field looks thermal to an accelerating observer. Given a thermal state, there is a pure state of a complete system which reduces to it. We find an analogous pure state for a Grand Canonical ensemble. We focus on a scalar complex field on a two-dimensional spacetime. Suppose the spacelike surface of the observer is the half infinite line. In this limit, we show that the entanglement entropy is the same as in the vacuum of a free field. The pure state that we study is the ground state of a scalar field in an imaginary Aharonov-Bohm potential. We use the Heat kernel regularization for effective action. We show that under appropriate boundary conditions, it regularizes the above-reduced density matrix. |