Source attack of decoy-state quantum key distribution using phase information

TL;DR

This paper demonstrates a source-side attack on decoy-state quantum key distribution systems that exploit phase information, emphasizing the importance of phase randomization for security.

Contribution

It introduces and experimentally verifies a novel attack exploiting source phase information in decoy-state QKD, highlighting a critical security loophole.

Findings

The attack can compromise systems without phase randomization.

Phase randomization is essential for secure decoy-state QKD.

The attack uses unambiguous-state-discrimination measurement.

Abstract

Quantum key distribution (QKD) utilizes the laws of quantum mechanics to achieve information-theoretically secure key generation. This field is now approaching the stage of commercialization, but many practical QKD systems still suffer from security loopholes due to imperfect devices. In fact, practical attacks have successfully been demonstrated. Fortunately, most of them only exploit detection-side loopholes which are now closed by the recent idea of measurement-device-independent QKD. On the other hand, little attention is paid to the source which may still leave QKD systems insecure. In this work, we propose and demonstrate an attack that exploits a source-side loophole existing in qubit-based QKD systems using a weak coherent state source and decoy states. Specifically, by implementing a linear-optics unambiguous-state-discrimination measurement, we show that the security of a system without phase randomization --- which is a step assumed in conventional security analyses but sometimes neglected in practice --- can be compromised. We conclude that implementing phase randomization is essential to the security of decoy-state QKD systems under current security analyses.