Entangled webs: Tight bound for symmetric sharing of entanglement
  Quantum entanglement cannot be unlimitedly shared among arbitrary number of qubits. Larger the number of entangled pairs in an N-qubit system, smaller the degree of bi-partite entanglement is. 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 |0> and the others in the basis state |1>. 
Masato Koashi (1), Vladimir Buzek (2), Nobuyuki Imoto (1) ((1)   Soken,Hayama,Japan, (2) Inst. Phys. SAS, Bratislava,Slovakia)