Entangled quantum photons react to the rotation of the Earth

A team of researchers led by Philip Walther at the University of Vienna performed a pioneering experiment where they measured the effect of the Earth’s rotation on entangled quantum photons. The work, published in Advances in sciencerepresents an important achievement that pushes the limits of spin sensing in entanglement-based sensors, potentially setting the stage for further exploration at the intersection between quantum mechanics and general relativity.

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

Interferometers using quantum entanglement have the potential to break those limits. If two or more particles become entangled, only the overall state is known, while the state of the individual particles 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 has been 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 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 along a closed rotational path arrive at the starting point at different times. With two entangled particles, it gets spooky: they behave as 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 present experiment, two entangled photons were propagating inside a 2-kilometer-long optical fiber wound into a large spiral, realizing an interferometer with an effective area of ​​more than 700 square meters.

A major hurdle the researchers faced was isolating and extracting the signal of the Earth’s steady rotation. “The crux of the matter lies in establishing a reference point for our measurement where the light remains unaffected by the rotating effect of the Earth. 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,” explains lead author Raffaele Silvestri.

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

The experiment, which was carried out as part of the TURIS research network organized by the University of Vienna and the Austrian Academy of Sciences, has 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 thousands-fold improvement in accuracy compared to previous experiments.

“This represents an important milestone as, 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,” says Haocun Yu, who worked on this experiment like Marie-Curie. Postdoctoral fellow.

“I believe our result and methodology will set the stage for further improvements in the spin sensitivity of entanglement-based sensors. This could pave the way for future experiments that test the behavior of quantum entanglement through space-time curves,” adds Philip Walther.