Phase randomization improves the security of quantum key distribution

TL;DR

This paper demonstrates that phase randomization is crucial for the security of quantum key distribution with weak coherent states, as knowledge of the phase can be exploited by eavesdroppers to compromise the key.

Contribution

The authors provide an explicit attack showing that lack of phase randomization can allow eavesdroppers to learn the entire key, emphasizing the importance of phase randomization for security.

Findings

Phase randomization enhances QKD security against phase knowledge attacks.

Explicit attack exploits phase information to break security in non-randomized protocols.

Security proofs must include source characterization, especially phase properties.

Abstract

Ideal quantum key distribution (QKD) protocols call for a source that emits single photon signals, but the sources used in typical practical realizations emit weak coherent states instead. A weak coherent state may contain more than one photon, which poses a potential security risk. QKD with weak coherent state signals has nevertheless been proven to be secure, but only under the assumption that the phase of each signal is random (and completely unknown to the adversary). Since this assumption need not be fully justified in practice, it is important to know whether phase randomization is really a requirement for security rather than a convenient technical assumption that makes the security proof easier. Here, we exhibit an explicit attack in which the eavesdropper exploits knowledge of the phase of the signals, and show that this attack allows the eavesdropper to learn every key bit in a parameter regime where a protocol using phase-randomized signals is provably secure. Thus we demonstrate that phase randomization really does enhance the security of QKD using weak coherent states. This result highlights the importance of a careful characterization of the source for proofs of the security of quantum key distribution.