Analysis of the Trusted‐Device Scenario in Continuous‐Variable Quantum Key Distribution

The assumption that detection and/or state preparation devices used for continuous‐variable quantum key distribution are beyond influence of potential eavesdroppers leads to a significant performance enhancement in terms of achievable key rate and transmission distance. A detailed and comprehensible derivation of the Holevo bound in this so‐called trusted‐device scenario is provided. Modeling an entangling‐cloner attack and using some basic algebraic matrix transformations, it is shown that the computation of the Holevo bound can be reduced to the solution of a quadratic equation. As an advantage of our derivation, the mathematical complexity of our solution does not increase with the number of trusted‐noise sources. Finally, a numerical evaluation of our results is provided, illustrating the counter‐intuitive fact that an appropriate amount of trusted‐receiver loss and noise can even be beneficial for the key rate.