Massimiliano Proietti

Quantum conference key agreement (CKA) is a cryptographic task for sharing a secret common key between multiple users. CKA has been established as a network protocol that can leverage multipartite entanglement (NQKD) to gain an advantage over contemporary two-party communication primitives (2QKD). Specifically, when performing QCKA in constrained quantum networks, e.g., with limited channel capacities, NQKD schemes can produce the conference key between N users with up to an N-1 rate advantage over 2QKD. QCKA has previously been implemented by direct transmission of a 4-photon GHZ state, however did not show the advantage comparison. Here we show this advantage using a universal network resource represented by a 6-qubit photonic graph state.

Paradigmatic QKD protocols establish secure keys between pairs of users, however when more than two parties want to communicate, recently introduced quantum conference quantum key agreement (CKA) protocols can outperform 2-party primitives in terms of resource cost. In this contribution we report an implementation of a four-user quantum CKA protocol using polarisation-encoded multi-partite GHZ states at telecom wavelength. We distribute these states over up to 50km of optical fibre and implement custom multiparty error correction and privacy amplification on the resulting raw keys. From a finite-key analysis, we establish an information-theoretic secure key of up to 1.15 × 10^6 bits, which is used to encrypt and securely share an image between the four users. Surpassing the previous maximum distance for GHZ state transmission by more than an order of magnitude, these results demonstrate the viability of network protocols relying on multi-partite-entanglement. Future applications beyond quantum CKA include entanglement-assisted remote clock-synchronization, quantum secret sharing, and GHZ-based repeater protocols.

We investigate three promising Quantum Digital Signature (QDS) protocols that can sign arbitrarily long documents with information-theoretic security. We re-derive the security proof of each scheme obtaining, where possible, tighter security bounds while closing security loopholes caused by using non-authenticated communication. In one case, we upgrade the QDS protocol to information-theoretic security by replacing NIST-recommended hash functions with universal hashing. We systematically compare the performance of the three protocols by optimizing over their parameters and deduce the protocol requiring the shortest signatures and the minimal number of preshared secret bits. Finally, we propose a multi-partite QDS protocol that retains the best features of the analyzed protocols while allowing for an arbitrary number of receivers.