The ideal wavelength for daylight free-space quantum key distribution

TL;DR

This paper models satellite-to-ground quantum channels to identify optimal wavelengths for daylight quantum key distribution, enabling continuous, high-rate quantum communication during daytime by exploiting low-sun-background spectral windows.

Contribution

It identifies the optimal wavelengths for daylight QKD, especially at Fraunhofer lines, and proposes a practical single-photon source based on hexagonal boron nitride.

Findings

Daylight QKD is feasible within Fraunhofer lines and near-infrared spectrum.

The highest secret key length is at the Hα Fraunhofer line.

A full adaptable model for different link scenarios is provided.

Abstract

Quantum key distribution (QKD) has matured in recent years from laboratory proof-of-principle demonstrations to commercially available systems. One of the major bottlenecks is the limited communication distance in fiber networks due to the exponential signal damping. To bridge intercontinental distances, low Earth orbit satellites transmitting the quantum signals over the atmosphere can be used. These free-space links, however, can only operate during the night, as the sunlight otherwise saturates the detectors used to measure the quantum states. For applying QKD in a global quantum internet with continuous availability and high data rates, operation during daylight is required. In this work, we model a satellite-to-ground quantum channel for different quantum light sources to identify the optimal wavelength for free-space QKD in ambient conditions. Daylight quantum communication is possible within the Fraunhofer lines or in the near-infrared spectrum, where the intrinsic background from the sun is comparably low. The highest annual secret key length considering the finite key effect is achievable at the H\textalpha\ Fraunhofer line. More importantly, we provide the full model that can be adapted in general to any other specific link scenario. We also propose a true single-photon source based on a color center in hexagonal boron nitride coupled to a microresonator that can implement such a scheme. Our results can also be applied in roof-to-roof scenarios and are therefore relevant for near-future quantum networks.