Entangled Quantum Key Distribution Over Two Free-Space Optical Links

TL;DR

This paper demonstrates a real-time entangled quantum key distribution system over two free-space optical links spanning 1,575 meters, achieving secure key generation with practical hardware and synchronization methods.

Contribution

It presents the first real-time implementation of entangled QKD over free-space links with a comprehensive system including polarization analysis, GPS synchronization, and complete protocol execution.

Findings

Average secure key rate of 85 bits/s

Quantum bit error rate of 4.92%

Operation over 6.5 hours during the night

Abstract

We report on the first real-time implementation of a quantum key distribution (QKD) system using entangled photon pairs that are sent over two free-space optical telescope links. The entangled photon pairs are produced with a type-II spontaneous parametric down-conversion source placed in a central, potentially untrusted, location. The two free-space links cover a distance of 435 m and 1,325 m respectively, producing a total separation of 1,575 m. The system relies on passive polarization analysis units, GPS timing receivers for synchronization, and custom written software to perform the complete QKD protocol including error correction and privacy amplification. Over 6.5 hours during the night, we observed an average raw key generation rate of 565 bits/s, an average quantum bit error rate (QBER) of 4.92%, and an average secure key generation rate of 85 bits/s.