  We calculate the entanglement entropy of a slab of finite width in the pure
Maxwell theory. We find that a large part of entropy is contributed by the
entanglement of a mode, nonlocal in terms of the transverse magnetic field
degrees of freedom. Even though the entangled mode is nonlocal, its
contribution to the entropy is local in the sense that the entropy of a slab of
a finite thickness is equal to the entropy of the boundary plus a correction
exponential in thickness of the slab.
