Entangled webs: Tight bound for symmetric sharing of entanglement

Quantum entanglement cannot be unlimitedly shared among an arbitrary number of qubits. The degree of bipartite entanglement decreases as the number of entangled pairs in an N-qubit system increases. We analyze a system of N qubits in which an arbitrary pair of particles is entangled. We show that the maximum degree of entanglement ~measured in the concurrence! between any pair of qubits is 2/N . This tight bound can be achieved when the qubits are prepared in a pure symmetric ~with respect to permutations! state with just one qubit in the basis state u0& and the others in the basis state u1&.