Long-distance quantum key distribution secure against coherent attacks

TL;DR

This paper demonstrates that the BB84 quantum key distribution protocol can securely generate positive key rates over long distances up to 240 km against coherent attacks, using practical detectors and multiplexing techniques.

Contribution

The study provides the first demonstration of long-distance QKD security against coherent attacks with practical detectors and multiplexing, extending secure communication distances.

Findings

Secure key rates up to 240 km without multiplexing

Secure key rates up to 200 km with multiplexing

Practical thermo-electrically cooled detectors enable long-distance QKD

Abstract

Quantum key distribution (QKD) permits information-theoretically secure transmission of digital encryption keys, assuming that the behaviour of the devices employed for the key exchange can be reliably modelled and predicted. Remarkably, no assumptions have to be made on the capabilities of an eavesdropper other than that she is bounded by the laws of Nature, thus making the security of QKD "unconditional". However, unconditional security is hard to achieve in practice. For example, any experimental realisation can only collect finite data samples, leading to vulnerabilities against coherent attacks, the most general class of attacks, and for some protocols the theoretical proof of robustness against these attacks is still missing. For these reasons, in the past many QKD experiments have fallen short of implementing an unconditionally secure protocol and have instead considered limited attacking capabilities by the eavesdropper. Here, we explore the security of QKD against coherent attacks in the most challenging environment: the long-distance transmission of keys. We demonstrate that the BB84 protocol can provide positive key rates for distances up to 240 km without multiplexing of conventional signals, and up to 200 km with multiplexing. Useful key rates can be achieved even for the longest distances, using practical thermo-electrically cooled single-photon detectors.