Relative Wavefront Errors in Continuous-Variable Quantum Communication

TL;DR

This paper provides experimental evidence that relative wavefront errors occur in atmospheric CV-QKD systems, especially over long-range links, potentially impacting secure key rates and challenging previous assumptions.

Contribution

It is the first to experimentally demonstrate the presence of relative wavefront errors in CV-QKD and analyze how turbulence influences these errors.

Findings

Relative wavefront errors are experimentally observed in atmospheric CV-QKD.

Turbulence significantly affects the form of these wavefront errors.

Long-range links are more impacted by turbulence-induced wavefront errors.

Abstract

When undertaking continuous-variable quantum key distribution (CV-QKD) across atmospheric channels, strong classical local oscillators (LOs) are often polarization-multiplexed with the weak quantum signals for coherent measurement at the receiver. Although the wavefronts of the quantum signal and LO are often assumed to experience the same distortion across channels, previous theoretical work has shown that they can experience differential distortions, resulting in relative wavefront errors (WFEs). Such errors have previously been shown to limit CV-QKD performance, in some cases leading to zero secure key rates. In this work, for the first time, we provide strong experimental evidence that relative WFEs are present in some circumstances and that standard assumptions in CV-QKD deployments may need to be revisited. In addition, we demonstrate how turbulence can affect the detailed form of the relative WFEs, thereby indicating that long-range links like terrestrial-satellite channels are likely impacted more than short-range terrestrial-only channels.