Concept
Quantum entanglement
8 studiesEvidence last moved Sep 27, 2026
Entanglement is a correlation between quantum systems that no classical model can reproduce, verified with Bell tests, witnesses or fidelity thresholds. These papers cover distributing entanglement over city-scale fibre, on chips via phonons, in multi-user networks, and using it for sensing and measurement.
Headlines describe 'quantum networks' and 'quantum sensors' as if they were ready; these experiments show what thresholds were actually passed, at what rates, and with what caveats. Students also often confuse super-resolution with better sensitivity, which the rotation-sensing work separates clearly.
Studies
8
Findings
5
8 supporting · 0 challenging · 0 qualifying citations
Open tensions
2
Latest change
Concept page published
Quantum entanglement
Currently
What we know
- City-scale links work, but slowly and near thresholds.
- Sound waves can carry quantum information on chip.
- Multiplexing saves channels in entanglement networks.
- Entanglement gives measurable advantages, in narrow settings.
- Special states and paths can control how entanglement evolves.
Largest unresolved question
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.
Common misconceptions
An entangled sensor is more sensitive than the best classical sensor.
The N00N-state gyroscope doubled the phase (super-resolution) but did not beat classical fibre gyroscopes in absolute sensitivity.
A fidelity above 0.5 means a link is useful for applications.
The heralded NV fidelity of 0.534 proves entanglement but is too low for most protocols, as the authors note.
Teleportation sends information faster than light or moves matter.
The 64 km experiment transferred a photon's quantum state using a Bell measurement and classical feedback, at about 7 events per second.
Related
Claim ledger
What the evidence shows
Drawn from 8 studies in this library. Mix labels say which citation roles are present; they are not a strength score. Supports means evidence for a finding; Challenges means evidence against a stated position; Qualifies marks scope.
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.
- Can quantum bits in two cities be entangled over real fibre?
- Can quantum teleportation run fast over city-scale fibre?
Study Role Design N Population Outcome Can quantum bits in two cities be entangled over real fibre? Supports OtherDeployed 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 Two qubit nodes; no single sample count Nitrogen-vacancy electron spin qubits in diamond at two nodes linked by deployed fibre Entangled-state fidelity and entanglement generation rate, postselected and fully heralded Can quantum teleportation run fast over city-scale fibre? Supports OtherThree-station fibre teleportation experiment (Alice, Bob, Charlie) with a PPLN entangled-photon source, Bell-state measurement, active timing and polarisation feedback, and state tomography. Physics experiment; results are photon coincidence counts, not a sample of units. Photonic time-bin qubits sent over deployed and spooled telecom fibre on a university campus Teleportation rate, teleportation fidelity (vs the classical limit), entanglement visibility and photon indistinguishability Sound waves can carry quantum information on chip.
Entanglement can be verified inside a chip with non-photonic carriers: a photon entangled with a travelling phonon qubit gave a CHSH value S = 2.32 ± 0.08, above the classical bound of 2.
Multiplexing saves channels in entanglement networks.
One microring source can serve several users: a dual-pumped silicon nitride ring gave Franson visibilities of 87-98% and secure keys for all six user pairs in a four-node lab network (1946.9 bps total, asymptotic).
Entanglement gives measurable advantages, in narrow settings.
Entangled probes improve measurements in specific ways: two-photon N00N states doubled the Earth-rotation phase in a fibre Sagnac interferometer (factor 1.96), and two-copy entangling measurements on an IBM device beat the single-copy error limit by 19 ± 4%.
- Can entangled photons measure the Earth's rotation?
- Can measuring two quantum copies together beat measuring them one by one?
Study Role Design N Population Outcome Can entangled photons measure the Earth's rotation? Supports OtherOptical-fibre Sagnac interferometer with 2 km of fibre on a rotatable frame, fed with single photons or two-photon N00N states, with an optical switch to turn the rotation signal on and off. No sample of units; phases extracted from photon-coincidence fringes (11 settings of 30-minute runs at the main angle) plus five further frame angles. Entangled photon pairs at 1546 nm propagating in a large fibre Sagnac interferometer Earth-rotation-induced Sagnac phase shift and entanglement enhancement factor Can measuring two quantum copies together beat measuring them one by one? Supports OtherTheory-designed measurement circuits run on several quantum processors (IBM, Rigetti, trapped-ion, photonic) to estimate two small qubit rotations under controlled decoherence No participant sample; each unknown angle was estimated 400 times, each estimate averaging 512 circuit repetitions (341 for three-copy circuits) Qubits on the Fraunhofer IBM Q System One, 11 cloud IBM Q processors, Rigetti Aspen-9, the AQTION trapped-ion processor and the JenQuant photonic processor Mean squared error of simultaneous estimates of two rotation angles, compared with Nagaoka and Holevo bounds Special states and paths can control how entanglement evolves.
Entanglement dynamics can be engineered: scar states on a superconducting qubit ladder showed slow entanglement growth and revivals lasting about a microsecond versus 50 ns decay for generic states, and loops around an exceptional point in a photonic walk switched Bell states with fidelities of at least 84%.
- Can special quantum states avoid scrambling and keep entanglement?
- Can looping around an exceptional point switch entangled states?
Study Role Design N Population Outcome Can special quantum states avoid scrambling and keep entanglement? Supports OtherTheory of exact scar eigenstates in a two-row qubit ladder, tested by quench dynamics and state tomography on a tunable-coupler superconducting processor, with numerical simulations including device imperfections No sample size; circuits of two rows with up to eight qubits each Transmon superconducting qubits in a ladder with tunable-sign couplings Population imbalance, subsystem fidelity revivals and entanglement entropy over time for special versus generic initial states Can looping around an exceptional point switch entangled states? Supports OtherNon-Hermitian quantum walk theory with a two-photon polarisation experiment reconstructed by quantum state tomography No sample N; the experiment used an 8-step quantum walk with four Bell-state inputs, two encircling directions, and ten random disorder realisations Polarisation-entangled photon pairs from type-I down-conversion in a BBO crystal Fidelity of output states to target Bell states and theory-experiment similarity
Debates
Tensions and limits
Some items are genuine disagreements on the same question. Others mark 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.
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.
- Can quantum bits in two cities be entangled over real fibre?
- Can quantum teleportation run fast over city-scale fibre?
Study Role Design N Population Outcome Can quantum bits in two cities be entangled over real fibre? Supports OtherDeployed 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 Two qubit nodes; no single sample count Nitrogen-vacancy electron spin qubits in diamond at two nodes linked by deployed fibre Entangled-state fidelity and entanglement generation rate, postselected and fully heralded Can quantum teleportation run fast over city-scale fibre? Supports OtherThree-station fibre teleportation experiment (Alice, Bob, Charlie) with a PPLN entangled-photon source, Bell-state measurement, active timing and polarisation feedback, and state tomography. Physics experiment; results are photon coincidence counts, not a sample of units. Photonic time-bin qubits sent over deployed and spooled telecom fibre on a university campus Teleportation rate, teleportation fidelity (vs the classical limit), entanglement visibility and photon indistinguishability
PaperFren reads this as a limit on how far one study travels — different assays, populations, or outcomes — not a forced fight between papers.
Scaling fails in practice: three-copy measurements performed worse than two-copy because of gate errors, and N00N states lost about 99% of pairs to loss, so advantages shown with two copies or two photons may not grow with size.
Scaling fails in practice: three-copy measurements performed worse than two-copy because of gate errors, and N00N states lost about 99% of pairs to loss, so advantages shown with two copies or two photons may not grow with size.
- Can measuring two quantum copies together beat measuring them one by one?
- Can entangled photons measure the Earth's rotation?
Study Role Design N Population Outcome Can measuring two quantum copies together beat measuring them one by one? Supports OtherTheory-designed measurement circuits run on several quantum processors (IBM, Rigetti, trapped-ion, photonic) to estimate two small qubit rotations under controlled decoherence No participant sample; each unknown angle was estimated 400 times, each estimate averaging 512 circuit repetitions (341 for three-copy circuits) Qubits on the Fraunhofer IBM Q System One, 11 cloud IBM Q processors, Rigetti Aspen-9, the AQTION trapped-ion processor and the JenQuant photonic processor Mean squared error of simultaneous estimates of two rotation angles, compared with Nagaoka and Holevo bounds Can entangled photons measure the Earth's rotation? Supports OtherOptical-fibre Sagnac interferometer with 2 km of fibre on a rotatable frame, fed with single photons or two-photon N00N states, with an optical switch to turn the rotation signal on and off. No sample of units; phases extracted from photon-coincidence fringes (11 settings of 30-minute runs at the main angle) plus five further frame angles. Entangled photon pairs at 1546 nm propagating in a large fibre Sagnac interferometer Earth-rotation-induced Sagnac phase shift and entanglement enhancement factor
PaperFren reads this as a limit on how far one study travels — different assays, populations, or outcomes — not a forced fight between papers.
Timeline
How understanding moved
Study years are when the paper was published. Evidence edits are dated changes to this page's claims. Explanations are when PaperFren added a Discovery — not a claim that the science happened that day.
2026
Concept page published
Quantum entanglement
Change log
What changed
Dated edits to this page's evidence: studies added or removed from a claim, claims added or withdrawn, and new explanations tagged here. Rewordings are not listed.
- Concept page published
Papers
8 studies in this library bear on Quantum entanglement, ordered by citations.
- 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.
- Can quantum teleportation run fast over city-scale fibre?
Researchers teleported single-photon quantum states across a campus fibre network several times per second with fidelity well above what any classical method could reach.
- Can measuring two quantum copies together beat measuring them one by one?
Measuring two copies of a noisy qubit jointly with an entangling circuit estimated two rotation angles more precisely than any one-at-a-time measurement can, on real quantum computers.
- Can sound waves carry entangled quantum states across a chip?
Single quanta of sound travelling along a tiny silicon waveguide were shown to carry entanglement, passing a Bell test with a partner photon.
- Can entangled photons measure the Earth's rotation?
Pairs of entangled photons sent around a giant fibre loop detected Earth's spin and picked up twice the phase shift that single photons did, as quantum theory predicts.
- Can special quantum states avoid scrambling and keep entanglement?
In a chaotic ladder of superconducting qubits, certain specially built states keep oscillating and stay structured instead of thermalizing, and the entanglement of one family of them can be dialled with disorder.
- Can looping around an exceptional point switch entangled states?
Steering a lossy photonic system around an exceptional point converts one entangled Bell state into another, with the result set by the loop's direction and robust to small errors.
- Can one wavelength carry entanglement to three partners at once?
Pumping a chip-scale ring resonator with two lasers let one wavelength channel be entangled with three others, halving the channels needed to fully connect four quantum-network users.
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Questions
What is still open
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.
Scaling fails in practice: three-copy measurements performed worse than two-copy because of gate errors, and N00N states lost about 99% of pairs to loss, so advantages shown with two copies or two photons may not grow with size.
Ask PaperFren about Quantum entanglement
Study this conceptflashcards and short-answer questions
What does a CHSH value of 2.32 tell you, and what does it not?
A CHSH S above 2 rules out local hidden-variable explanations, so the photon and travelling phonon were entangled; here S = 2.32 ± 0.08, about 4 standard deviations above 2. It does not show long-distance or useful distribution: phonons travelled only millimetres, the dataset took 56 hours, and only one device was used.
Compare the N00N-state gyroscope result with the idea of 'quantum advantage'.
Two-photon N00N states produced a phase shift 1.96 times larger than single photons, matching the predicted factor of two. This is super-resolution. But losses removed about 99% of photon pairs, and classical gyroscopes remain more sensitive. The advantage is real in principle but not practical in this setup.
Flashcards
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