ABOUT OPTIMAL CLONING AND ENTANGLEMENT
The main non-classical feature of quantum key distribution (QKD) is that it is characterized by a trade-off relation that limits the information possibly gained by a spy and the quality of the transmission line between the authorized users. In particular, perfect cloning is impossible, due to this trade-off, while optimal imperfect cloning saturates the trade-off relation. We investigate by numerical methods the deep nature of this trade-off relation, in the case of optimal cloning, and find that it reveals a subtle interplay between fidelity and entanglement.
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