Joseph Ho

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 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.

Here we report on the experimental results implementing robust anonymous quantum conference key agreement using GHZ states. Results confirm the advantage when allowing for the use of multipartite entanglement along with bipartite entanglement.

Quantum state elimination measurements tell us what states a quantum system does not have. This is different from state discrimination, where one tries to determine what the state of a quantum system is, rather than what it is not. Apart from being of fundamental interest, quantum state elimination may find uses in quantum communication and quantum cryptography. We consider unambiguous elimination of a pair of quantum states, and present a possible optical realisation of the scheme.