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A team of researchers led by Philipp Walter of the University of Vienna performed a pioneering experiment in which they measured the effect of the Earth’s rotation on quantum entangled photons. The work published in Scientific progressrepresents a significant achievement that extends the limits of spin sensitivity in entanglement-based sensors, potentially setting the stage for further exploration of the intersection between quantum mechanics and general relativity.

Optical Sagnac interferometers are the most sensitive devices to rotations. They have been key to our understanding of fundamental physics since the early years of the last century, contributing to the establishment of Einstein’s special theory of relativity. Today, their unmatched precision makes them the best tool for measuring rotational speeds, limited only by the limits of classical physics.

Interferometers using quantum entanglement have the potential to break these limits. If two or more particles are entangled, only the overall state is known, while the state of the individual particle remains undetermined until measurement. This can be used to obtain 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. This is where the Vienna experiment made the difference.

The researchers built a giant Sagnac fiber optic interferometer and kept the noise low and stable for several hours. This enabled the detection of sufficiently high-quality entangled photon pairs to surpass the rotational precision of previous quantum optical Sagnac interferometers by a factor of a thousand.

In a Sagnac interferometer, two particles traveling in opposite directions on a rotating closed trajectory reach the origin at different times. With two entangled particles, it gets spooky: they behave like a single particle testing both directions at the same time, while accumulating twice the time delay compared to the no-entanglement scenario.

This unique property is known as super resolution. In the actual experiment, two entangled photons propagated inside a 2-kilometer-long optical fiber wound on a huge coil, realizing an interferometer with an effective area of ​​more than 700 square meters.

A significant hurdle facing researchers is isolating and extracting the signal of the Earth’s permanent rotation. “The heart of the matter lies in establishing a reference point for our measurement where the light remains unaffected by the Earth’s rotational effect. Given our inability to stop the Earth’s rotation, we devised a workaround: splitting the optical fiber into two coils of equal length and connecting them via an optical switch,” explains lead author Raffaele Silvestri.

By flipping the switch on and off, the researchers could effectively cancel the spin signal at will, which also allowed them to extend the stability of their large apparatus. “We’ve essentially tricked light into thinking it’s in a non-rotating universe,” says Silvestri.

The experiment, which was conducted as part of the TURIS research network organized by the University of Vienna and the Austrian Academy of Sciences, successfully observed the effect of the Earth’s rotation on the maximally entangled two-photon state. This confirms the interplay between rotating reference frames 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 because a century after the first observation of the Earth’s rotation with light, the entanglement of individual light quanta finally entered the same modes of sensitivity,” says Haocun Yu, who worked on this experiment as Maria -Curie Postdoctoral Fellow.

“I believe our result and methodology will lay the foundation for further improvements in the rotational sensitivity of entanglement-based sensors.” This could pave the way for future experiments testing the behavior of quantum entanglement through the curvature of spacetime,” adds Philip Walter.