Can quantum bits in two cities be entangled over real fibre?
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.
Source
Metropolitan-scale heralded entanglement of solid-state qubits
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.
What they did
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.
What they found
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.
The limits
What it doesn't show
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.
Key terms
- Heralded entanglement
- Entanglement whose successful creation is signalled by a detection event, so the users know when a usable entangled state exists.
- NV centre
- A nitrogen atom next to a missing carbon in diamond whose electron spin acts as a qubit and can emit photons.
- Quantum frequency conversion
- Shifting a single photon's wavelength, here from 637 nm to a telecom band, while preserving its quantum state, to reduce fibre loss.
- Single-click protocol
- An entangling scheme where detection of one photon after a beam splitter heralds entanglement; its rate scales with the square root of transmission.
- Fidelity
- How close a produced quantum state is to the ideal target state; above 0.5 for a Bell state proves entanglement.
Flashcards
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Quiz yourself
Why were NV photons converted to about 1588 nm?
Common questions
Why convert the photons to a telecom wavelength?
Optical fibre absorbs much less light in the telecom bands, so converting the photons reduces loss over kilometres of fibre.
What is the difference between postselected and heralded entanglement?
In postselection the qubits are measured immediately and entanglement is identified afterwards, so it cannot be reused; heralded delivery tells the nodes in real time, leaving a live entangled state.
Why is the heralded rate so much lower?
Each attempt must wait for the herald signal to travel to the nodes and be processed, making attempts roughly twenty times slower.
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