Fundamental limitations to key distillation from Gaussian states with Gaussian operations

We establish fundamental upper bounds on the amount of secret key that can be extracted from quantum Gaussian states by using local Gaussian operations, local classical processing, and public communication. For one-way public communication or when two-way public communication is allowed but Alice and Bob first perform destructive local Gaussian measurements, we prove that the key is bounded by the Renyi-2 Gaussian entanglement of formation EGF,2. The saturation of this inequality for pure Gaussian states provides an operational interpretation of the Renyi-2 entropy of entanglement as the secret key rate of pure Gaussian states accessible with Gaussian operations and one-way communication. In the general setting of two-way communication and arbitrary interactive protocols, we argue that 2EGF,2 still serves as an upper bound on the extractable key. We conjecture that the factor of 2 is spurious, suggesting that EGF,2 coincides with the secret key rate of Gaussian states under Gaussian measurements and two-way public communication. We use these results to prove a gap between the secret key rates obtainable with arbitrary versus Gaussian operations. This gap is observed for states produced by sending one half of a two-mode squeezed vacuum through a pure loss channel, in the regime of sufficiently low squeezing or sufficiently high transmissivity. Finally, for a wide class of Gaussian states, including all two-mode states, we prove a recently proposed conjecture on the equality between EGF,2 and the Gaussian intrinsic entanglement. The unified entanglement quantifier emerging from such an equality is then endowed with a direct operational interpretation as the value of a quantum teleportation game.

Automated summary

The article establishes upper bounds on the amount of secret key that can be extracted from quantum Gaussian states using local Gaussian operations, local classical processing, and public communication. The authors prove that the key is bounded by the Renyi-2 Gaussian entanglement of formation EGF,2 for certain communication scenarios. They also propose that this bound coincides with the secret key rate of Gaussian states under specific conditions. Additionally, the authors demonstrate a gap between the secret key rates obtainable with arbitrary versus Gaussian operations.