Quantum key distribution in the Holevo limit

A theorem by Shannon and the Holevo theorem impose that the efficiency of any protocol for quantum key distribution, $\cal E$, defined as the number of secret (i.e., allowing eavesdropping detection) bits per transmitted bit plus qubit, is ${\cal E} \le 1$. The problem addressed here is whether the limit ${\cal E} =1$ can be achieved. It is showed that it can be done by splitting the secret bits between several qubits and forcing Eve to have only a sequential access to the qubits, as proposed by Goldenberg and Vaidman. A protocol with ${\cal E} =1$ based on polarized photons and in which Bob's state discrimination can be implemented with linear optical elements is presented.Comment: REVTeX, 4 pages, 2 figure

Simple explanation of the quantum violation of a fundamental inequality

We show that the maximum quantum violation of the Klyachko-Can-Binicioglu-Shumovsky (KCBS) inequality is exactly the maximum value satisfying the following principle: The sum of probabilities of pairwise exclusive events cannot exceed 1. We call this principle "global exclusivity," since its power shows up when it is applied to global events resulting from enlarged scenarios in which the events in the inequality are considered jointly with other events. We identify scenarios in which this principle singles out quantum contextuality, and show that a recent proof excluding nonlocal boxes follows from the maximum violation imposed by this principle to the KCBS inequality.Comment: REVTeX4, 6 pages, 3 figure