Go Kato

Quantum key distribution (QKD) promises theoretically unbreakable encryption by exploiting the principles of quantum mechanics. However, the security of real-world implementations is compromised by inevitable device imperfections, unless these are accounted for in the security proof. In this work, we introduce an innovative and powerful security proof framework that guarantees robustness against all practical source imperfections while maintaining high performances, thereby significantly bridging the gap between the theoretical promise and practical realization of QKD. In combination with measurement-device-independent QKD, which closes all security loopholes related to the measurement units, our framework can guarantee an unprecedented level of implementation security.

The key relay protocol (KRP) plays an important role in improving the performance and the security of quantum key distribution (QKD) networks. On the other hand, there is also an existing research field called secure network coding (SNC), which has similar goal and structure. We here analyze differences and similarities between the KRP and SNC rigorously. We found, rather surprisingly, that there is a definite gap in security between the KRP and SNC; that is, certain KRPs achieve better security than any SNC schemes on the same graph. We also found that this gap can be closed if we generalize the notion of SNC by adding free public channels; that is, KRPs are equivalent to SNC schemes augmented with free public channels.

Quantum networks will allow to implement communication tasks beyond the reach of their classical counterparts. A pressing and necessary issue for the design of quantum network protocols is the quantification of the rates at which these tasks can be performed. Here, we propose a simple recipe that yields efficiently computable lower and upper bounds on the maximum achievable rates. For this we make use of the max-flow min-cut theorem and its generalization to multi-commodity flows to obtain linear programs. We exemplify our recipe deriving the linear programs for bipartite settings, settings where multiple pairs of users obtain entanglement in parallel as well as multipartite settings, covering almost all known situations. We also make use of a generalization of the concept of paths between user pairs in a network to Steiner trees spanning a group of users wishing to establish Greenberger-Horne-Zeilinger states.

In most of the security proofs of continuous variable quantum key distribution (CV-QKD), except for the security proof of an entangle-based protocol by F. Furrer et. al. [Phys. Rev. Lett. 109 , 100502 (2012)], it is required that Bob’s local oscillator (LO) for homodyne or heterodyne measurements is perfectly prepared. Since Eve can freely manipulate LO transmitted from Alice to Bob in standard CV-QKD systems, this requirement cannot be met. Moreover, the requirement includes the assumption that the intensity of Bob’s LO must be infinite, which is impossible to achieve in reality. In this work, we fill the gap between a standard CV-QKD system and the existing security proofs by providing a security proof accommodating the manipulation of LO by Eve.