Quantum Key Distribution with State Replacement

Quantum key distribution protocols have been studied for almost four decades, and have already seen real world applications. Such implementations often operate in naturally noisy environments (e.g., sensor thermal noise, sun radiation, etc.) with known or measurable noise levels. However, existing security proofs implicitly attribute any channel noise to a possible eavesdropper, an overly pessimistic assumption that can adversely effect the guaranteed protocol key-rate. In this paper, we analyze general prepare-and-measure QKD schemes, and show how to improve the bounds on the key-rate under the assumption of an independent state replacement noise at the receiver. Taking BB84 as a concrete example, our results theoretically guarantee a higher key-rate, as a function of the minimal possible replacement probability. In practice, for some parameter-regimes, this guarantees a higher tolerable QBER.