Quantum entangled photons respond to the Earth’s rotation

A team of researchers led by Philip Walther from 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 advancesrepresents a significant achievement that pushes the limits of spin sensitivity in entanglement-based sensors and potentially paves the way for further exploration at the intersection between quantum mechanics and general relativity.

Optical Sagnac interferometers are the most sensitive devices for rotation. Since the early years of the last century, they have been central to our understanding of fundamental physics and contributed to the creation of Einstein’s special theory of relativity. Their unmatched accuracy makes them the ultimate tool for measuring rotation rates today, limited only by the limits of classical physics.

Interferometers using quantum entanglement have the potential to break these boundaries. If two or more particles are entangled, only the overall state is known, while the state of an individual particle remains undetermined until measurement. This can be used to get more information per measurement than would be possible without it. However, the promised quantum leap in sensitivity is hindered by the extremely fine nature of the entanglement. Here is where the Vienna experiment made a difference.

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

In a Sagnac interferometer, two particles moving in opposite directions rotating closed paths reach the starting point at different times. With two entangled particles, things get spooky: they behave as one particle testing both directions simultaneously, accumulating twice the time delay compared to the scenario where no entanglement is present.

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 giant coil, creating an interferometer with an effective area of ​​more than 700 square meters.

A significant hurdle the researchers faced was isolating and extracting the Earth’s steady rotation signal. “The heart of the matter is to establish a reference point for our measurements where the light remains unaffected by the Earth’s rotation effect. Given our inability to stop the Earth’s rotation, we came up with a solution: splitting the optical fiber into two coils of equal length and connecting them via an optical switch,” explains the principal by Raffaele Silvestri.

By turning the switch on and off, the researchers could effectively cancel the rotation signal at will, which also allowed them to extend the stability of their large apparatus. “Essentially, we tricked the light into thinking it was 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, successfully observed the effect of the Earth’s rotation on the maximally entangled two-photon state. This confirms the interaction 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 a significant milestone, because a hundred years after the first observation of the Earth’s rotation with light, the entanglement of individual light quanta has finally reached the same modes of sensitivity,” says Haocun Yu, who worked on the experiment as a Marie-Curie. Postdoctoral fellow.

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