Long-distance free-space quantum key distribution with continuous variables

TL;DR

This paper demonstrates long-distance free-space quantum key distribution using continuous variables over 7 km inland and 9.6 km maritime channels, overcoming atmospheric noise and daylight challenges, advancing satellite-based quantum communication.

Contribution

It introduces a novel high-precision manipulation and acquisition technology enabling outdoor CVQKD over long distances without extra wavelength filtering, surpassing previous indoor or short-range experiments.

Findings

Successful 7 km inland and 9.6 km maritime CVQKD over atmospheric channels.

Overcomes atmospheric noise and daylight challenges without wavelength conversion.

System is compatible with existing fiber networks, enabling integrated air-ground quantum networks.

Abstract

Continuous-variable quantum key distribution (CVQKD) enables remote users to share high-rate and unconditionally secure secret keys while maintaining compatibility with classical optical communication networks and effective resistance against background noise. However, CVQKD experiments have only been demonstrated indoors or over short outdoor distances. Here, by developing channel-fluctuation-independent high-precision manipulation of continuous-variable quantum states, high-accuracy quantum signal acquisition and processing, and high-efficiency free-space acquisition, tracking, and pointing technology, we overcome the excess noise due to atmospheric effects especially in daylight without extra wavelength conversion and spectral filtering, and demonstrate for the first time long-distance free-space quantum key distribution over 7-km inland and 9.6-km maritime atmospheric channels with Gaussian-modulated coherent states. This achieved distribution distance of secure quantum secret keys is well beyond the atmosphere's effective thickness, offering a promising alternative for realizing satellite-based quantum cryptography communication in daylight. Moreover, given that the CVQKD system is naturally compatible with existing ground fiber telecommunication networks, it marks an essential step for realizing integrated air-ground quantum access networks with cross-domain applications.