Composable end-to-end security of Gaussian quantum networks with untrusted relays

TL;DR

This paper develops a security framework for Gaussian quantum networks with untrusted nodes, providing a parameter estimation method and demonstrating potential for surpassing repeaterless limits with quantum amplifiers.

Contribution

It introduces a general methodology for composable security in Gaussian quantum networks with untrusted nodes, applicable in finite-size regimes, and explores quantum amplifier-assisted chains.

Findings

Key rate can surpass repeaterless limit with quantum amplifiers

Parameter estimation relies solely on data shared by end-users

Practical challenges motivate new network/chain designs

Abstract

Gaussian networks are fundamental objects in network information theory. Here many senders and receivers are connected by physically motivated Gaussian channels while auxiliary Gaussian components, such as Gaussian relays, are entailed. Whilst the theoretical backbone of classical Gaussian networks is well established, the quantum analogue is yet immature. Here, we theoretically tackle composable security of arbitrary Gaussian quantum networks (quantum networks), with generally untrusted nodes, in the finite-size regime. We put forward a general methodology for parameter estimation, which is only based on the data shared by the remote end-users. Taking a chain of identical quantum links as an example, we further demonstrate our study. Additionally, we find that the key rate of a quantum amplifier-assisted chain can ideally beat the fundamental repeaterless limit with practical block sizes. However, this objective is practically questioned leading the way to new network/chain designs.