Delft three-node NV-centre network

What it is

Each of the three nodes is an NV centre in diamond — a nitrogen atom adjacent to a lattice vacancy — used as the communication qubit, with a nearby nuclear spin as a robust memory qubit and local quantum-gate primitive. Alice ↔ Bob and Bob ↔ Charlie share phase-stabilised optical links; entanglement is heralded between neighbour pairs, then Bob performs a Bell-state measurement on his two communication qubits to swap the entanglement so Alice and Charlie end up sharing a Bell pair. ^{Pompili et al. 2021}

The 2022 follow-up upgraded the protocol to deterministic qubit teleportation between non-neighbours: Alice teleports an arbitrary unknown qubit state to Charlie using only locally-shared Bell pairs and classical communication through Bob, with no direct optical link between Alice and Charlie. ^{Hermans et al. 2022} That is the canonical quantum-internet primitive — sending an unknown qubit through a Bell pair plus two classical bits — running end-to-end on a real three-node fabric.

Verified claims

• First entanglement-based network with three nodes. Published in Science 372.259 (2021). The "first multinode quantum network of remote solid-state qubits" framing is the authors' own. ^{Pompili et al. 2021}
• Entanglement distribution + entanglement swapping at Bob. Both protocols ran on the platform in the 2021 paper. ^{Pompili et al. 2021}
• 2022 follow-up: qubit teleportation between Alice and Charlie. Reported by Hermans et al., Nature 605.663 (2022). First end-to-end teleportation between non-neighbouring nodes. ^{Hermans et al. 2022}
• NV-centre platform with auxiliary nuclear-spin memory qubits. The robust memory was essential for the swap protocol to succeed against the heralding-rate / memory-decoherence trade-off. ^{Pompili et al. 2021}

Things to note

• Setting. The three nodes sit in the same building, with the optical links running on a lab bench. The contribution is architectural rather than geographic.
• Rates. Heralded entanglement between two NV centres runs at the 1–10 Hz scale; end-to-end teleportation rates in 2022 are lower still. The platform demonstrates the protocols rather than throughput.
• Scope. The Delft fabric is a kernel demonstration that the building blocks compose; it is not yet offered as a service to other QuTech labs or outside users.
• Relation between the two papers. The 2022 teleportation result uses the 2021 swap as a subroutine. Both papers describe the same physical platform as sequential milestones on the same setup.

Related entries

• Entanglement and Bell pairs — the resource being distributed.
• Entanglement swapping — the Bob-side protocol used to link Alice and Charlie.
• Quantum teleportation — the 2022 follow-up's demonstrated protocol.
• Quantum memories — the NV / nuclear-spin combination Delft uses.
• Quantum repeaters — the architecture this is a kernel demonstration of.
• Knaut Boston-metro demo — sibling matter-qubit deployment, on installed urban fibre.