Information theoretic security of quantum key distribution overcoming the repeaterless secret key capacity bound

TL;DR

This paper introduces a new quantum key distribution method that uses decoy states and mode switching to surpass the traditional secret key capacity limit, enabling secure long-distance quantum communication.

Contribution

It proposes a novel decoy state protocol with mode switching that overcomes the secret key capacity bound in quantum key distribution.

Findings

Overcomes the secret key capacity limit at long distances

Confirms the scaling properties of the proposed scheme

Enables practical secure quantum communication

Abstract

Quantum key distribution is a way to distribute secret keys to distant users with information theoretic security and key rates suitable for real-world applications. Its rate-distance figure, however, is limited by the natural loss of the communication channel and can never surpass a theoretical limit known as point-to-point secret key capacity. Recently, a new type of quantum key distribution with an intermediate relay was proposed to overcome this limit (M. Lucamarini, Z. L. Yuan, J. F. Dynes and A. J. Shields, Nature, 2018). However, a standard application of the decoy state method limited the security analysis of this scheme to hold under restrictive assumptions for the eavesdropper. Hence, overcoming the point-to-point secret key capacity with an information-theoretic secure scheme is still an open question. Here, we propose a novel way to use decoy states to answer this question. The key idea is to switch between a Test mode and a Code mode, the former enabling the decoy state parameter estimation and the latter generating a key through a phase encoding protocol. This way, we confirm the scaling properties of the original scheme and overcome the secret key capacity at long distances. Our work plays a key role to unlock the potential of practical secure quantum communications.