Security Analysis of an Untrusted Source for Quantum Key Distribution: Passive Approach

TL;DR

This paper introduces a passive security analysis method for quantum key distribution with untrusted sources, eliminating the need for active routing or random number generators, and demonstrates its practicality through simulations and experiments.

Contribution

It provides a complete proof of unconditional security for a passive scheme that simplifies real-world QKD implementations with untrusted sources.

Findings

Passive estimation remains effective despite imperfections

Simulations confirm robustness against losses and noise

Preliminary experiments validate practical utility

Abstract

We present a passive approach to the security analysis of quantum key distribution (QKD) with an untrusted source. A complete proof of its unconditional security is also presented. This scheme has significant advantages in real-life implementations as it does not require fast optical switching or a quantum random number generator. The essential idea is to use a beam splitter to split each input pulse. We show that we can characterize the source using a cross-estimate technique without active routing of each pulse. We have derived analytical expressions for the passive estimation scheme. Moreover, using simulations, we have considered four real-life imperfections: Additional loss introduced by the "plug & play" structure, inefficiency of the intensity monitor, noise of the intensity monitor, and statistical fluctuation introduced by finite data size. Our simulation results show that the passive estimate of an untrusted source remains useful in practice, despite these four imperfections. Also, we have performed preliminary experiments, confirming the utility of our proposal in real-life applications. Our proposal makes it possible to implement the "plug & play" QKD with the security guaranteed, while keeping the implementation practical.