Fundamental limitations to key distillation from Gaussian states with Gaussian operations
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We establish fundamental upper bounds on the amount of secret key that can be extracted from continuous variable quantum Gaussian states by using only local Gaussian operations, local classical processing, and public communication. For one-way communication, we prove that the key is bounded by the Rényi-2 Gaussian entanglement of formation E_F,2^ G, with the inequality being saturated for pure Gaussian states. The same is true if two-way public communication is allowed but Alice and Bob employ protocols that start with destructive local Gaussian measurements. In the most general setting of two-way communication and arbitrary interactive protocols, we argue that 2 E_F,2^ G is still a bound on the extractable key, although we conjecture that the factor of 2 is superfluous. Finally, for a wide class of Gaussian states that includes all two-mode states, we prove a recently proposed conjecture on the equality between E_F,2^ G and the Gaussian intrinsic entanglement, thus endowing both measures with a more solid operational meaning.
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