End-to-End QKD Using LEO Satellite Networks

TL;DR

The results show that the achievable key rates scale favourably with constellation size, with Type-II constellations reaching operational continuity and generating multi-gigabit secret keys per day, demonstrating a practical route toward secure global quantum communication.

Abstract

We propose a satellite-based Quantum Key Distribution (QKD) network that enables global-scale, end-to-end secure key exchange without relying on trusted intermediate nodes. The network is formed by a ring constellation of satellites that maintain persistent inter-satellite connectivity and support two configurations: a polar Type-I constellation providing global coverage, and an equatorial Type-II constellation offering continuous, terrestrial-like operation. End-to-end secrecy is achieved through the use of Twin-field Quantum Key Distribution (TF-QKD) and a redundant XOR-based key-forwarding protocol, in which each forwarding step incorporates independently generated QKD keys from ground-satellite and inter-satellite links. As a result, the final secret key is never exposed to any intermediate satellite, eliminating the single-point vulnerabilities inherent in trusted-node networks. Scaling the network offers two benefits: improved security and higher key rates. Increasing the constellation size enhances security by forcing an adversary to compromise a larger number of nodes to break the protocol, while simultaneously improving link availability and key throughput. Using realistic uplink and Inter-Satellite Link (ISL) models, we compute finite-size secret-key lengths based on the Sending-or-not-sending (SNS)-TF-QKD protocol. Our results show that the achievable key rates scale favourably with constellation size, with Type-II constellations reaching operational continuity and generating multi-gigabit secret keys per day, demonstrating a practical route toward secure global quantum communication.

Figures (6)

• Figure 1: Illustration of the Type-1 constellation, consisting of equally spaced polar orbital planes with one satellite per plane. This geometry ensures continuous inter-satellite line-of-sight, thereby maintaining an unbroken ring network at all times. In this configuration, $GS_1$ and $GS_2$ are assumed to lie at the same latitude but on opposite sides of the Earth (a longitudinal separation of $180^\circ$).
• Figure 2: Illustration of the Type-2 constellation, consisting of an equatorial orbit with all satellites evenly placed in the orbit. This geometry ensures continuous inter-satellite and offers a high visibility ratio. In this configuration, $GS_1$ and $GS_2$ are assumed to lie at the same latitude but on opposite sides of the Earth (a longitudinal separation of $180^\circ$).
• Figure 3: Schematics of the satellite-based network. Two ground stations, $GS_1$ and $GS_2$, connect with the ring constellation of satellites. Secret keys $X^+$ and $X^-$ are established along the two directed ring segments, $\Gamma^+$ and $\Gamma^-$, respectively. Blue arrows indicate links between satellites and between satellites and ground stations, while the red arrow denotes the point-to-point links between ground stations and satellites.
• Figure 4: Uplink channel loss as a function of the satellite pass. For the ground-to-satellite uplink, a cutoff zenith angle of 70◦ is imposed. Under this constraint, a zenith pass ( $\theta = 0^\circ$) yields a maximum ground–satellite QKD session duration of 294 seconds.
• Figure 5: loss between $S_i$ and $S_{i+1}$ for the Type-I (polar) constellation. Increasing the number of satellites reduces the inter-satellite spacing, thereby lowering the optical loss. The shaded region indicates the range of uplink losses observed during a ground–satellite session. For comparison, the losses at the equator correspond to the Type-II (equatorial) constellation.