Janka Memmen

We report on an experimental demonstration of a recently proposed (n, n)-QSS (quantum secret sharing) protocol, which can be shown to be secure against participant attacks, using a four-photon GHZ state. Our work leverages the generation of high-quality and high-brightness non-linear single photon sources to achieve a secure key rate of 745 bits/sec in the asymptotic regime marking an important step toward scalable quantum-secure communication in networks.

Whilst the use of multi-partite entanglement is known to offer an advantage over bi-partite protocols in certain contexts, the quest to find practical advantage scenarios is ongoing and substantial difficulties in generalising some bi-partite security proofs remain. We present rigorous results that address both these challenges. First, we prove the security of a variant of the GHZ state based secret sharing protocol against general attacks, including participant attacks which break the security of the original GHZ state scheme. We then identify parameters for a performance advantage over realistic bottleneck networks in terms of extractable secret bits per network use. We show that whilst channel losses limit the advantage region to short distances over direct transmission networks, the addition of quantum repeaters unlocks the performance advantage of multi-partite entanglement over point-to-point approaches for long distance quantum cryptography.