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Physicists at the University of Vienna have experimentally measured the rotation rate of our planet using maximally path-entangled quantum states of light in a large-scale interferometer.
Silvestri et al. demonstrated the largest and most precise quantum-optical Sagnac interferometer to date, exhibiting sufficient sensitivity to measure Earth’s rotation rate. Image credit: Marco Di Vita.
For more than a century, interferometers have been important instruments to experimentally test fundamental physical questions.
They disproved the luminiferous ether, helped establish special relativity and enabled the measurement of tiny ripples in space-time itself known as gravitational waves.
With recent advances in technology, interferometers can nowadays also operate using various different quantum systems including electrons, neutrons, atoms, superfluids, and Bose-Einstein condensates.
“If two or more particles are entangled, only the overall state is known, while the state of the individual particle remains undetermined until measurement,” said co-lead author Dr. Philip Walther and his colleagues.
“This can be used to obtain more information per measurement than would be possible without it.”
“However, the promised quantum leap in sensitivity has been hindered by the extremely delicate nature of entanglement.”
For their research, the authors built a giant optical fiber Sagnac interferometer and kept the noise low and stable for several hours.
This enabled the detection of enough high-quality entangled photon pairs such to outperform the rotation precision of previous quantum optical Sagnac interferometers by a thousand times.
“In a Sagnac interferometer, two particles traveling in opposite directions of a rotating closed path reach the starting point at different times,” the researchers explained.
“With two entangled particles, it becomes spooky: they behave like a single particle testing both directions simultaneously while accumulating twice the time delay compared to the scenario where no entanglement is present.”
“This unique property is known as super-resolution.”
In the experiment, two entangled photons were propagating inside a 2-km-long optical fiber wounded onto a huge coil, realizing an interferometer with an effective area of more than 700 m2.
A significant hurdle the team faced was isolating and extracting Earth’s steady rotation signal.
“The core of the matter lays in establishing a reference point for our measurement, where light remains unaffected by Earth’s rotational effect,” said Dr. Raffaele Silvestri, first author of the study.
“Given our inability to halt Earth’s from spinning, we devised a workaround: splitting the optical fiber into two equal-length coils and connecting them via an optical switch.”
“By toggling the switch on and off we could effectively cancel the rotation signal at will, which also allowed us to extend the stability of their large apparatus.”
“We have basically tricked the light into thinking it’s in a non-rotating Universe.”
The team successfully observed the effect of the rotation of Earth on a maximally entangled two-photon state.
This confirms the interaction between rotating reference systems and quantum entanglement, as described in Einstein’s special theory of relativity and quantum mechanics, with a thousand-fold precision improvement compared to previous experiments.
“That represents a significant milestone since, a century after the first observation of Earth’s rotation with light, the entanglement of individual quanta of light has finally entered the same sensitivity regimes,” said co-lead author Dr. Haocun Yu.
“I believe our result and methodology will set the ground to further improvements in the rotation sensitivity of entanglement-based sensors.”
“This could open the way for future experiments testing the behavior of quantum entanglement through the curves of spacetime,” Dr. Walther said.
The team’s work appears in the journal Science Advances.
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Raffaele Silvestri et al. 2024. Experimental observation of Earth’s rotation with quantum entanglement. Science Advances 10 (24); doi: 10.1126/sciadv.ado0215
