Multi-partite quantum cryptographic protocols with noisy GHZ States

We propose a wide class of distillation schemes for multi-partite entangled states that are CSS-states. Our proposal provides not only superior efficiency, but also new insights on the connection between CSS-states and bipartite graph states. We then apply our distillation schemes to the tri-partite case for three cryptographic tasks|namely, (a) conference key agreement, (b) quantum sharing of classical secrets and (c) third-man cryptography. Moreover, we construct "prepare-and-measure" protocols for the above three cryptographic tasks which can be implemented with the generation of only a single entangled pair at a time. This gives significant simplification over previous experimental implementations which require two entangled pairs generated simultaneously. We also study the yields of those protocols and the threshold values of the fidelity above which the protocols can function securely. Rather surprisingly, our protocols will function securely even when the initial state does not violate the standard Bell-inequalities for GHZ states.