Passive-scheme analysis for solving untrusted source problem in quantum key distribution

TL;DR

This paper analyzes passive monitoring schemes in quantum key distribution to address untrusted source issues, demonstrating that photon number analyzers outperform average photon number monitors and approach the ideal performance of photon number distribution monitors.

Contribution

The paper provides a rigorous security analysis of passive schemes in QKD, comparing APN, PNA, and PND monitors, and shows PNA's practical advantages under realistic conditions.

Findings

PNA outperforms APN in security and efficiency

PNA can approach PND performance asymptotically

Passive scheme with PNA is effective when signal-to-noise ratio is adequate

Abstract

As a practical method, the passive scheme is useful to monitor the photon statistics of an untrusted source in a "Plug & Play" quantum key distribution (QKD) system. In a passive scheme, three kinds of monitor mode can be adopted: average photon number (APN) monitor, photon number analyzer (PNA) and photon number distribution (PND) monitor. In this paper, the security analysis is rigorously given for APN monitor, while for PNA, the analysis including statistical fluctuation and random noise, is addressed with a confidence level. The results show that the PNA can achieve better performance than the APN monitor and can asymptotically approach the theoretical limit of the PND monitor. Also, the passive scheme with the PNA works efficiently when the signal-to-noise ratio ($R^{SN}$) is not too low and so is highly applicable to solve the untrusted source problem in the QKD system.