  We discuss the nonlocal nature of quantum mechanics and the link with
relativistic quantum mechanics such as formulated by quantum field theory. We
use here a nonlocal quantum field theory (NLQFT) which is finite, satisfies
Poincar\'e invariance, unitarity and microscopic causality. This nonlocal
quantum field theory associates infinite derivative entire functions with
propagators and vertices. We focus on proving causality and discussing its
importance when constructing a relativistic field theory. We formulate scalar
field theory using the functional integral in order to characterize quantum
entanglement and the entanglement entropy of the theory. Using the replica
trick, we compute the entanglement entropy for the theory in 3 + 1 dimensions
on a cone. The result is free of UV divergences and we recover the area law.
