Fully-Passive Quantum Key Distribution

TL;DR

This paper introduces a fully-passive quantum key distribution source using linear optics, eliminating active modulators for decoy-state and encoding, enhancing practicality and security against side-channels.

Contribution

It proposes a novel, fully-passive QKD source with linear optics that supports multiple protocols without active modulators, improving security and practicality.

Findings

Creates arbitrary qubit states passively

Compatible with various QKD protocols

Eliminates side-channels from modulators

Abstract

Passive implementations of quantum key distribution (QKD) sources are highly desirable as they eliminate side-channels that active modulators might introduce. Up till now, passive decoy-state and passive encoding BB84 schemes have both been proposed. Nonetheless, passive decoy-state generation and passive encoding have never been simultaneously implemented with linear optical elements before, which greatly limits the practicality of such passive QKD schemes. In this work, we overcome this limitation and propose a fully-passive QKD source with linear optics that eliminates active modulators for both decoy-state choice and encoding. This allows for highly practical QKD systems that avoid side-channels from the source modulators. The passive source we propose (combined with the decoy-state analysis) can create any arbitrary state on a qubit system and is protocol-independent. That is, it can be used for various protocols such as BB84, reference-frame-independent QKD, or the six-state protocol. It can also in principle be combined with e.g. measurement-device-independent QKD, to build a system without side-channels in either detectors or modulators.