Upper Bounds for the Security of two Distributed-Phase Reference Protocols of Quantum Cryptography (Coherent-One-Way and Differential-Phase-Shift)

TL;DR

This paper establishes new upper bounds on the security of two practical quantum cryptography protocols, DPS and COW, over long distances, considering collective attacks and proposing more robust protocol variants.

Contribution

It introduces novel security upper bounds for DPS and COW protocols against collective attacks and proposes modified versions of COW for enhanced robustness.

Findings

Security bounds applicable to distances over 50 km in optical fibers

Analysis of collective attacks attaching ancillary quantum systems

Proposal of modified COW protocols with improved robustness

Abstract

The Differential-Phase-Shift (DPS) and the Coherent-One-Way (COW) are among the most practical protocols for quantum cryptography, and are therefore the object of fast-paced experimental developments. The assessment of their security is also a challenge for theorists: the existing tools, that allow to prove security against the most general attacks, do not apply to these two protocols in any straightforward way. We present new upper bounds for their security in the limit of large distances ($d \gtrsim 50$km with typical values in optical fibers) by considering a large class of collective attacks, namely those in which the adversary attaches ancillary quantum systems to each pulse or to each pair of pulses. We introduce also two modified versions of the COW protocol, which may prove more robust than the original one.