  We show for a general pure entangled state of two two-level atoms, the von
Neumann entropy of the partial traces can be directly measured from the
magnitude of the mean spin vector of a single atom of the pair. We emphasize
the fact that the von Neumann entropy of the partial traces for such a system
can be obtained without knowing the exact form of the quantum state of the two
atoms, if we have the value of the magnitude of the mean spin vector of a
single atom of the pair. Mean spin vector, used in the context of spin
squeezing and spectroscopic squeezing in population spectroscopy, is
experimentally measurable and provides an exact measure of von Neumann entropy
of the partial traces for such a system. The idea developed in this paper can
be used in the context of other quantum mechanical two level systems as the
algebra of two level systems can be described by that of spin- $\frac{1}{2}$
particles.
