# Quantum secret sharing using weak coherent states

## TL;DR

This paper introduces a continuous variable quantum secret sharing protocol using coherent states, which is scalable, immune to Trojan horse attacks, and secure under high channel loss conditions.

## Contribution

It proposes a novel continuous variable QSS scheme with practical laser sources, demonstrating unconditional security and scalability unlike previous entanglement-based protocols.

## Key findings

- Protocol is immune to Trojan horse attacks.
- Secure under high channel loss conditions.
- Scalable to many players with minimal additional loss.

## Abstract

Secret sharing allows a trusted party (the dealer) to distribute a secret to a group of players, who can only access the secret cooperatively. Quantum secret sharing (QSS) protocols could provide unconditional security based on fundamental laws in physics. While the general security proof has been established recently in an entanglement-based QSS protocol, the tolerable channel loss is unfortunately rather small. Here we propose a continuous variable QSS protocol using conventional laser sources and homodyne detectors. In this protocol, a Gaussian-modulated coherent state (GMCS) prepared by one player passes through the secure stations of the other players sequentially, and each of the other players injects a locally prepared, independent GMCS into the circulating optical mode. Finally, the dealer measures both the amplitude and phase quadratures of the receiving optical mode using double homodyne detectors. Collectively, the players can use their encoded random numbers to estimate the measurement results of the dealer and further generate a shared key. Unlike the existing single photon based sequential QSS protocol, our scheme is intrinsically immune to Trojan horse attacks. Furthermore, the additional loss introduced by each player's system can be extremely small, which makes the protocol scalable to a large number of players. We prove the unconditional security of the proposed protocol against both eavesdroppers and dishonest players in the presence of high channel loss, and discuss various practical issues.