Quantum key distribution with flawed and leaky sources

TL;DR

This paper extends the security analysis of quantum key distribution (QKD) to more realistic scenarios with device imperfections and multiple optical modes, providing guidelines for secure practical implementations.

Contribution

It generalizes the loss-tolerant protocol to include multiple optical modes and removes the qubit assumption, enhancing practical security analysis of QKD.

Findings

Security parameters identified for secure QKD with flawed sources

Comparison of key rates shows optimal method depends on device imperfections

Guidelines for characterizing QKD transmitters established

Abstract

In theory, quantum key distribution (QKD) allows secure communications between two parties based on physical laws. However, most of the security proofs of QKD today make unrealistic assumptions and neglect many relevant device imperfections. As a result, they cannot guarantee the security of the practical implementations. Recently, the loss-tolerant protocol (K. Tamaki et al, Phys. Rev. A, 90, 052314, 2014) was proposed to make QKD robust against state preparation flaws. This protocol relies on the emission of qubit systems which, unfortunately, is difficult to achieve in practice. In this work, we remove such qubit assumption and generalise the loss-tolerant protocol to accommodate multiple optical modes in the emitted signals. These multiple optical modes could arise, for example, from Trojan horse attacks and/or device imperfections. Our security proof determines some dominant device parameter regimes needed for achieving secure communication, and therefore it can serve as a guideline to characterise QKD transmitters. Furthermore, we compare our approach with that of Lo and Preskill (H.-K. Lo et al, Quantum Inf. Comput., 7, 431-458, 2007) and identify which method provides the highest secret key generation rate as a function of the device imperfections. Our work constitutes an important step towards the best practical and secure implementation for QKD.