  We evaluate the vacuum entanglement entropy across a cut of future null
infinity for free Maxwell theory in four-dimensional Minkowski spacetime. The
Weyl invariance of 4D Maxwell theory allows us to embed the Minkowski spacetime
inside the Einstein static universe. The Minkowski vacuum can then be described
as a thermofield double state on the (future) Milne wedges of the original and
an inverted Minkowski patches. We show that the soft mode contribution to
entanglement entropy is due to correlations between asymptotic charges of these
Milne wedges, or equivalently nontrivial conformally soft (or edge) mode
configurations at the entangling surface.
