Surpassing the repeaterless bound with a photon-number encoded measurement-device-independent quantum key distribution protocol

TL;DR

This paper introduces a high-dimensional, measurement-device-independent quantum key distribution protocol that surpasses the fundamental repeaterless bound, enabling longer secure communication distances with current technology.

Contribution

The authors propose a novel protocol using high-dimensional states and coherent photon number detection, outperforming existing protocols and surpassing the repeaterless bound.

Findings

Outperforms existing measurement-device-independent QKD protocols.

Surpasses the fundamental repeaterless bound for pure-loss channels.

Achieves longer transmission distances considering experimental imperfections.

Abstract

Decoherence is detrimental to quantum key distribution (QKD) over large distances. One of the proposed solutions is to use quantum repeaters, which divide the total distance between the users into smaller segments to minimise the effects of the losses in the channel. However, the secret key rates that repeater protocols can achieve are fundamentally bounded by the separation between each neighbouring node. Here we introduce a measurement-device-independent protocol which uses high-dimensional states prepared by two distant trusted parties and a coherent total photon number detection for the entanglement swapping measurement at the repeater station. We present an experimentally feasible protocol that can be implemented with current technology as the required states reduce down to the single-photon level over large distances. This protocol outperforms the existing measurement-device-independent and twin-field QKD protocols by surpassing the fundamental limit of the repeaterless bound for the pure-loss channel at a shorter distance and achieves a higher transmission distance in total when experimental imperfections are considered.