Quantum conference key agreement

Short answer: Quantum conference key agreement (QCKA) lets more than two parties establish a common secret key from a shared multipartite entangled state. A Science Advances paper reports distributing four-photon GHZ states over up to 50 km of fibre and establishing 1.5 million bits of secure key under finite-key analysis, and an npj Quantum Information paper reports a more than two-fold key-rate enhancement for GHZ states over a pairwise approach in a six-user photonic testbed. These are laboratory network demonstrations.

The task

A review in Advanced Quantum Technologies describes conference key agreement (CKA), also called multipartite key distribution, as a task in which more than two parties wish to establish a common secret key. It generalizes key distribution to N users, and the common key lets users broadcast secure messages in a network. The review says CKA can be achieved by first establishing bipartite keys between the users and then distributing a common key through them, and that this has been discussed as inefficient in the classical scenario. It says that in the quantum scenario a composition of bipartite quantum key distribution links can do the job, but that multipartite quantum correlations allow new and potentially more efficient protocols. The review covers protocols based on multipartite entanglement, in both the device-dependent and device-independent settings.

The GHZ advantage

The npj Quantum Information paper says QCKA can be implemented efficiently in a quantum network using a single copy of an N-qubit Greenberger-Horne-Zeilinger (GHZ) state to distil a secure N-user conference key bit, whereas up to N-1 entanglement pairs are consumed in the traditional pairwise protocol. The authors report a testbed of a photonic six-user quantum network in which four users can distil either a GHZ state or the required number of Bell pairs using network routing techniques. In the asymptotic limit they report a more than two-fold enhancement of the conference key rate for the GHZ protocol. They extrapolate their data to show that the resource advantage persists when finite-key effects are taken into account.

Distribution over fibre

The Science Advances authors report demonstrating QCKA with up to an N-1 rate advantage in constrained networks. They distribute four-photon GHZ states, generated by high-brightness telecom photon-pair sources, over optical fibre with combined lengths of up to 50 km, then perform multiuser error correction and privacy amplification. Under finite-key analysis they report establishing 1.5 million bits of secure key, which they used to encrypt and share an image between four users.

Limits

The advantage is one of resource use in a constrained network, and the figures come from specific testbeds with four to six users. The authors describe a path to future networks, not a deployed service. Bipartite quantum links are, according to the review, already being implemented in small networks over metropolitan distances, which is a separate matter from multiparty links.

FAQ

What is conference key agreement?

A task in which more than two parties establish a common secret key, per the Advanced Quantum Technologies review.

Why use GHZ states?

The npj Quantum Information paper says one GHZ state can yield an N-user key bit, where the pairwise protocol consumes up to N-1 entanglement pairs.

How much better was it?

More than two-fold in the asymptotic limit in the six-user testbed, according to the authors.

How far did the photons travel?

Combined fibre lengths of up to 50 km in the Science Advances experiment.

Is it deployed?

No. These are laboratory testbed demonstrations.