Entangled webs: Tight bound for symmetric sharing of entanglement
Masato Koashi, Vladimír Bužek, Nobuyuki Imoto · Physical Review A · 2000
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 $|0〉$ and the others in the basis state $|1〉.$