Quantum Relays for Long Distance Quantum Cryptography

TL;DR

This paper explores quantum relays that split communication channels to extend the feasible distance for quantum cryptography, potentially enabling secure communication over hundreds of kilometers with current technology.

Contribution

It demonstrates that quantum relays can significantly extend the distance of quantum cryptography and introduces the use of reverse key reconciliation for improved security over longer distances.

Findings

Quantum relays can enable cryptography over hundreds of kilometers.

Reverse key reconciliation extends secure communication distance.

Standard schemes are sufficient for shorter distances.

Abstract

Quantum Cryptography is on the verge of commercial application. One of its greatest limitations is over long distance - secret key rates are low and the longest fibre over which any key has been exchanged is currently 100 km. We investigate the quantum relay, which can increase the maximum distance at which quantum cryptography is possible. The relay splits the channel into sections, and sends a different photon across each section, increasing the signal to noise ratio. The photons are linked as in teleportation, with entangled photon pairs and Bell measurements. We show that such a scheme could allow cryptography over hundreds of kilometers with today's detectors. It could not, however, improve the rate of key exchange over distances where the standard single section scheme already works. We also show that reverse key reconciliation, previously used in continuous variable quantum cryptography, gives a secure key over longer distances than forward key reconciliation.