  Entanglement is a key resource of quantum science for tasks that require it
to be shared among participants. Within atomic, condensed matter and photonic
many-body systems the distribution and sharing of entanglement is of particular
importance for information processing by progressively larger and larger
quantum networks. Here we report a singly-bipartitioned qubit entanglement
inequality that applies to any N-party qubit pure state and is completely
tight. It provides the first prescription for a direct calculation of the
amount of entanglement sharing that is possible among N qubit parties. A
geometric representation of the measure is easily visualized via polytopes
within entanglement hypercubes.
