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.
Source
Experimental observation of Earth's rotation with quantum entanglement
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
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What they did
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.
What they found
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.
The limits
What it doesn't show
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.
Key terms
- Sagnac effect
- In a rotating loop, light travelling with and against the rotation takes different times, creating a phase difference proportional to rotation rate and enclosed area.
- N00N state
- An entangled state where N photons are all in one path or all in the other, in superposition; it accumulates phase N times faster.
- Super-resolution
- Interference fringes oscillating N times faster than with single photons, due to multiphoton entanglement.
- Heisenberg limit
- The best possible phase precision in quantum measurement, scaling as 1/N rather than the standard quantum limit's 1/sqrt(N).
- Spontaneous parametric down-conversion
- A nonlinear crystal process that splits one pump photon into a pair of lower-energy photons, used to make entangled pairs.
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Quiz yourself
What enhancement factor from entanglement was measured?
Common questions
Why use an optical switch rather than just rotating the whole setup?
Earth's rotation can't be turned off, so zeroing the effective area gives a self-referenced 'no-signal' measurement that cancels drifts and technical noise.
Does this make the best rotation sensor ever?
No. It is the largest and most sensitive quantum-optical Sagnac interferometer, but classical fibre gyroscopes are still far more precise.
Why is the phase doubled for two photons?
A two-photon N00N state behaves like a single particle with twice the energy, so it picks up twice the Sagnac phase.
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