Physicists measure Earth’s spin using quantum entanglement | Scientific news

Physicists at the University of Vienna have experimentally measured the rotation rate of our planet using maximally entangled quantum states of light on the way in a large-scale interferometer.

For more than a century, interferometers have been important instruments for experimentally testing fundamental physical questions.

They disproved the luminous aether, helped create special relativity, and enabled the measurement of tiny ripples in spacetime itself known as gravitational waves.

With recent advances in technology, interferometers nowadays can also operate using various 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 particles remains undetermined until measurement,” said co-author Dr. Philip Walther and his colleagues.

“This can be used to get more measurement information than would be possible without it.”

“However, the promised quantum leap in sensitivity is hampered by the extremely delicate nature of entanglement.”

For their research, the authors built a giant Sagnac fiber optic interferometer and kept the noise low and stable for several hours.

This enabled the detection of enough high-quality entangled photon pairs to exceed the spin accuracy of previous optical quantum Sagnac interferometers by a factor of a thousand.

“In a Sagnac interferometer, two particles traveling in opposite directions of a closed rotational path reach the starting point at different times,” the researchers explained.

“With two entangled particles, it gets spooky: they behave like a single particle testing both directions simultaneously, while accumulating twice the time delay compared to the no-entanglement scenario.”

“This unique property is known as super-resolution.”

In the experiment, two entangled photons were propagating inside a 2 km long optical fiber wound into a large spiral, realizing an interferometer with an effective area of ​​more than 700 m2.

A significant hurdle the team faced was isolating and extracting the signal of Earth’s steady rotation.

“The crux of the matter lies in establishing a benchmark for our measurement where the light remains unaffected by the rotating effect of the Earth,” said Dr. Raffaele Silvestri, first author of the study.

“Given our inability to stop the Earth’s rotation, we devised a solution: splitting the optical fiber into two coils of equal length and connecting them via an optical switch.”

“By turning the switch on and off, we could effectively cancel the spin signal at will, which also allowed us to extend the robustness of their large apparatus.”

“We’ve basically tricked the light into thinking it’s in a non-rotating Universe.”

The team successfully observed the effect of the Earth’s rotation in a maximally entangled two-photon state.

This confirms the interplay between rotating reference systems and quantum entanglement, as described in Einstein’s special theory of relativity and quantum mechanics, with a thousandfold improvement in accuracy compared to previous experiments.

“This represents an important milestone since, a century after the first observation of the Earth’s rotation with light, the entanglement of individual light quanta has finally entered the same sensitivity regimes,” said lead author Dr. Haocun Yu.

“I believe our result and methodology will set the stage for further improvements in the rotation sensitivity of interlacing-based sensors.”

“This could pave the way for future experiments that test the behavior of quantum entanglement through the curvature of spacetime,” said Dr. Walther.

The team’s work appears in the journal Advances in science.

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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