Entanglement and quantum information
Can entangled photons measure the Earth's rotation?
Open access · cc by · source: Europe PMC
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.
Study at a glance
- Design
- Other — Optical-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.
- N
- 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.
- Population
- Entangled photon pairs at 1546 nm propagating in a large fibre Sagnac interferometer
- Outcome
- Earth-rotation-induced Sagnac phase shift and entanglement enhancement factor
Structured fields used in claim comparison tables when every cited study has a complete layer.
Key findings
With two-photon entangled states the Earth-induced phase shift was 5.5 mrad, versus 2.8 mrad for single photons, giving an enhancement factor of 1.96, consistent with the predicted factor of two. The phase followed the expected cosine dependence on frame angle, and both measurements gave an Earth rotation rate close to the accepted 7.3 × 10^-5 rad/s. The authors say this beats earlier entanglement-based rotation sensors by three orders of magnitude.
Methodology
The team wound 2 km of optical fibre on a rotatable 1.4 m square frame to make a Sagnac interferometer with an effective area of 715 square metres. They sent in either heralded single photons or two-photon N00N states, in which both photons travel together one way or the other, and measured interference fringes. An optical switch reversed half the loop to make the effective area zero, giving a built-in 'rotation off' reference, and they repeated measurements at several frame orientations.
Limitations
The entangled measurement shows super-resolution (a doubled phase) but not better absolute sensitivity than classical fibre gyroscopes, which already reach far finer resolution. N00N states are extremely fragile to loss: with about 90% of light lost in the setup, roughly 99% of photon pairs were lost, limiting scaling to more photons. Precision was also limited by vibration, thermal and acoustic noise, and the interferometer's scale factor was calibrated by assuming Earth's known rotation rate.
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.
Visibility thresholds, not fringe patterns alone, prove quantumness.
Nonclassicality is shown by beating classical thresholds: a metasurface gave a Hong-Ou-Mandel dip of 86% (classical limit 50%) and two-photon fringes of 86.8% visibility (threshold 70.7%), and fibre N00N states doubled a rotation phase.
Evidence for the claim as stated.
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%.
Evidence for the claim as stated.
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.
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.
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.
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