Abstract
We present a passive approach to the security analysis of quantum key distribution (QKD) with an untrusted source. A complete proof of its unconditional security is also presented. This scheme has significant advantages in real-life implementations as it does not require fast optical switching or a quantum random number generator. The essential idea is to use a beam splitter to split each input pulse. We show that we can characterize the source using a cross-estimate technique without active routing of each pulse. We have derived analytical expressions for the passive estimation scheme. Moreover, using simulations, we have considered four real-life imperfections: additional loss introduced by the 'plug&play' structure, inefficiency of the intensity monitor noise of the intensity monitor, and statistical fluctuation introduced by finite data size. Our simulation results show that the passive estimate of an untrusted source remains useful in practice, despite these four imperfections. Also, we have performed preliminary experiments, confirming the utility of our proposal in real-life applications. Our proposal makes it possible to implement the 'plug&play' QKD with the security guaranteed, while keeping the implementation practical.
GENERAL SCIENTIFIC SUMMARY Introduction and background. Quantum key distribution (QKD) enables unconditionally secure communication between two parties in the presence of sophisticated eavesdroppers. Every QKD system has a light source(s), which is typically assumed to be precisely characterized by legitimate users. However, this assumption often fails in practice due to the difficulty of precisely characterizing a light source. Moreover, in many QKD implementations, such as the 'plug & play' scheme (which is adopted by most commercial QKD systems) and some QKD network schemes, light sources are accessible and may even be manipulated by eavesdroppers. It is important to analyze the security of a QKD system whose light source is not well characterized by the users and may be compromised by an eavesdropper.
Main results. We present a passive approach to analyze the security of QKD with an untrusted source. A complete proof of its unconditional security is developed, in which we show that a few simple modifications are sufficient to guarantee the security of a QKD system with an untrusted source. We have considered four real-life imperfections: additional loss introduced by the 'plug & play' structure, inefficiency of the intensity monitor, noise of the intensity monitor, and statistical fluctuation introduced by finite data size. Our simulation results show that our analysis remains useful in practice, despite these four imperfections.
Wider implications. Our security analysis removes the impractical requirements/assumptions regarding the light source, allowing secure QKD even when the adversary has control over the light source.