Self-stabilized high-dimensional quantum key distribution on a metropolitan free-space link

TL;DR

This paper demonstrates a self-stabilized, high-dimensional quantum key distribution system over a hybrid metropolitan free-space and fiber link, achieving stable, long-term operation without auxiliary channels and enabling practical quantum networks.

Contribution

It introduces a fully self-referenced, high-dimensional QKD system operating continuously over 48 hours on a hybrid metropolitan link, advancing autonomous quantum communication.

Findings

Achieved secure key rates of 95 kbit/s for 4D and 59 kbit/s for 2D protocols.

Maintained continuous operation over 48 hours without auxiliary references.

Demonstrated adaptability of encoding dimensionality to channel conditions.

Abstract

Quantum communication technologies capable of operating reliably across heterogeneous optical channels are essential for scalable metropolitan quantum networks. Here we demonstrate high-dimensional time-bin-encoded quantum key distribution over a hybrid metropolitan link comprising 1.7 km free-space transmission and 685 m of optical fiber. Operating at a clock rate of 500 MHz in the C-band, we implement both 2- and 4-dimensional protocols, and obtain estimated secure finite-key rates of (95 +- 28) kbit/s for 4D at (25.0 +- 2.0) dB loss and (59 +- 27) kbit/s for 2D at (23.5 +- 2.3) dB loss. Crucially, we achieve continuous operation over 48 h in a fully self-referenced architecture: initial synchronization, interferometric phase stabilization, and long-term drift compensation are performed exclusively using the detected quantum signals, without auxiliary optical reference channels. Our results thus establish a practical and versatile platform for hybrid free-space-to-fiber quantum communication and show that the encoding dimensionality can be adapted to the optimal operating regime of realistic metropolitan channels, providing a pathway toward efficient, autonomous and deployable quantum network nodes.