Curator's Take
This article tackles a crucial real-world problem in quantum communication: how atmospheric turbulence and environmental fluctuations affect quantum channels in practical deployments like satellite-based quantum internet links. The research reveals a surprising silver lining – even when transmissivity varies dramatically due to fading conditions, quantum key distribution and entanglement sharing can still maintain positive rates as long as the channel isn't completely destroyed. Perhaps most intriguingly, the work shows that the standard thermal states typically used for bosonic quantum communication are actually suboptimal for these realistic fading scenarios, with specially crafted non-Gaussian states delivering superior performance. This finding could reshape how we design encoding strategies for future quantum networks operating in challenging real-world environments.
— Mark Eatherly
Summary
The pure-loss channel is a fundamental model for describing noise in bosonic quantum platforms. It is characterised by a single parameter, the transmissivity, which quantifies the fraction of the input energy that reaches the output of the channel. In realistic scenarios, however, such as free-space quantum communication, the transmissivity is not fixed but fluctuates from one channel use to another. In this setting, the overall channel is effectively described as a convex combination of pure-loss channels, known as a fading channel. Despite its practical relevance, the quantum Shannon theory of the fading channel has remained largely unexplored. Here, we address this gap, specifically investigating degradability, anti-degradability, entanglement breakingness, and capacities of the fading channel. Of particular relevance to practical quantum-internet applications, we prove that entanglement distribution and quantum key distribution can always be achieved at a strictly positive rate over any fading channel, no matter how noisy it is or how strongly the transmissivity fluctuates, provided the channel is not completely noisy. Moreover, we prove that thermal states, which are optimal for a broad class of static bosonic Gaussian channels, fail to achieve the entanglement-assisted classical capacity of fading channels: non-Gaussian Fock-diagonal states strictly outperform all Gaussian encodings. Most strikingly, we identify regimes where the coherent information of thermal inputs vanishes, while optimized non-Gaussian states achieve strictly positive values, thereby activating the channel for quantum communication. For a paradigmatic binary fading model we establish this result analytically, deriving the exact capacity-achieving state in closed form. For general fading distributions, we design an iterative variational algorithm to optimize the coherent and mutual information.