Entanglement and quantum information
Can quantum bits in two cities be entangled over real fibre?
Open access · cc by · source: Europe PMC
Two diamond-based quantum bits 10 kilometres apart were entangled through ordinary deployed telecom fibre, with the success signalled in real time so the shared state was ready to use.
Study at a glance
- Design
- Other — Deployed quantum network link: two NV-centre nodes (Delft, The Hague) joined via a midpoint heralding station using the single-click protocol, frequency conversion and multi-loop phase stabilization
- N
- Two qubit nodes; no single sample count
- Population
- Nitrogen-vacancy electron spin qubits in diamond at two nodes linked by deployed fibre
- Outcome
- Entangled-state fidelity and entanglement generation rate, postselected and fully heralded
Structured fields used in claim comparison tables when every cited study has a complete layer.
Key findings
Postselected entangled states had fidelities clearly above 0.5, proving entanglement, at rates up to 0.48 Hz for a 20-nanosecond detection window. In the fully heralded mode, real-time feedback delivered the same Bell state regardless of which detector clicked, with a fidelity of 0.534 at a much lower rate because each attempt had to wait for the herald to travel back. A parameter-free model reproduced the observed trade-off between rate and fidelity.
Methodology
Each node held a nitrogen-vacancy centre in diamond whose emitted photons were converted to a telecom wavelength and sent to a midpoint station, where interference and single-photon detection heralded entanglement. To make distant photons indistinguishable, the team actively stabilized timing, polarization, frequency and optical phase using light time-shared on the same fibre. They first measured entanglement in postselection, then ran a fully heralded protocol where the detection result was sent back to the nodes and used for real-time correction.
Limitations
The heralded fidelity of 0.534 is only just above the 0.5 entanglement threshold, too low for most practical protocols, and the delivery rate was about one success per minute. The link connected only two nodes, and the authors' projections of fidelities above 80% or 90% with better emitters or pulse sequences are model estimates, not measurements. Much of the stabilization detail and error budgeting is in supplementary materials not included here.
How this study connects
Role on claims
Each row is a claim on a concept or method page where this paper supports, challenges, or qualifies the statement. Roles are hand-checked — not a model guess.
City-scale links work, but slowly and near thresholds.
Entanglement can be delivered across metropolitan distances: two NV-centre nodes about Delft-The Hague apart were heralded into entangled states (fidelity 0.534 in the fully heralded mode), and fibre teleportation over 64 km reached about 7 Hz with fidelity above the classical 2/3 limit.
Evidence for the claim as stated.
Rate versus fidelity: the NV link trades rate for fidelity (0.48 Hz postselected, about one per minute heralded at 0.534), while photonic teleportation runs faster but used attenuated laser pulses, estimating single-photon fidelity with decoy states.
Evidence for the claim as stated.
Open questions
Tensions this paper is part of
From concept pages' “where studies disagree.” Disagreement means the same question; scope means different assays, populations, or outcomes.
Rate versus fidelity: the NV link trades rate for fidelity (0.48 Hz postselected, about one per minute heralded at 0.534), while photonic teleportation runs faster but used attenuated laser pulses, estimating single-photon fidelity with decoy states.
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Same topic cluster — not a recommendation engine.
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