# Beating the fundamental rate-distance limit in a proof-of-principle quantum key distribution system

## TL;DR

This paper demonstrates the first experimental implementation of twin-field quantum key distribution (TF-QKD), successfully surpassing the fundamental linear rate-distance limit at 300 km, highlighting its potential for long-distance secure communication.

## Contribution

It presents the first practical realization of a modified TF-QKD protocol that exceeds the linear bound without quantum repeaters, with high-visibility interference over 300 km.

## Key findings

- Successfully beat the linear rate-distance limit at 300 km
- Achieved high-visibility single-photon interference
- Demonstrated stable, high-rate measurement-device-independent QKD

## Abstract

With the help of quantum key distribution (QKD), two distant peers are able to share information-theoretically secure key bits. Increasing key rate is ultimately significant for the applications of QKD in lossy channel. However, it has proved that there is a fundamental rate-distance limit, named linear bound, which limits the performance of all existing repeaterless protocols and realizations. Surprisingly, a recently proposed protocol, called twin-field (TF) QKD can beat linear bound with no need of quantum repeaters. Here, we present the first implementation of TF-QKD protocol and demonstrate its advantage of beating linear bound at the channel distance of 300 km. In our experiment, a modified TF-QKD protocol which does not assume phase post-selection is considered, and thus higher key rate than the original one is expected. After well controlling the phase evolution of the twin fields travelling hundreds of kilometers of optical fibres, the implemented system achieves high-visibility single-photon interference, and allows stable and high-rate measurement-device-independent QKD. Our experimental demonstration and results confirm the feasibility of the TF-QKD protocol and its prominent superiority in long distance key distribution services.

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Source: https://tomesphere.com/paper/1902.06884