Long-distance free-space measurement-device-independent quantum key distribution

TL;DR

This paper demonstrates the first free-space measurement-device-independent quantum key distribution over a 19.2-km urban atmospheric channel, overcoming atmospheric turbulence challenges and paving the way for satellite-based quantum communication.

Contribution

It introduces a robust adaptive optics system and synchronization techniques enabling free-space MDI-QKD over long distances, surpassing previous atmospheric limitations.

Findings

Successful free-space MDI-QKD over 19.2 km urban channel

Overcomes atmospheric turbulence with adaptive optics

Lays groundwork for satellite-based quantum networks

Abstract

Measurement-device-independent quantum key distribution (MDI-QKD), based on two-photon interference, is immune to all attacks against the detection system and allows a QKD network with untrusted relays. Since the MDI-QKD protocol was proposed, fibre-based implementations have been rapidly developed towards longer distance, higher key rates, and network verification. However, owing to the effect of atmospheric turbulence, MDI-QKD over free-space channel remains experimentally challenging. Here, by developing the robust adaptive optics system, high precision time synchronization and frequency locking between independent photon sources located far apart, we realised the first free-space MDI-QKD over a 19.2-km urban atmospheric channel, which well exceeds the effective atmospheric thickness. Our experiment takes the first step towards satellite-based MDI-QKD. Moreover, the technology developed here opens the way to quantum experiments in free space involving long-distance interference of independent single photons.