Entangled quantum photons respond to Earth’s rotation

A team of researchers led by Philip Walther from the University of Vienna carried out a pioneering experiment in which they measured the effect of the Earth’s rotation on entangled quantum photons. The work, published in Scientists progressrepresents a significant achievement that pushes the boundaries of rotation sensitivity in entanglement-based sensors, potentially paving the way for further exploration at the intersection between quantum mechanics and general relativity.

Sagnac optical interferometers are the devices most sensitive to rotations. They have played a central role in our understanding of fundamental physics since the early years of the last century, helping to establish Einstein’s special theory of relativity. Today, their unrivaled 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 these limits. If two or more particles are entangled, only the overall state is known, while the state of each individual particle remains undetermined until measurement. This can be used to obtain more information per measurement than would otherwise be possible. However, the promised quantum leap in sensitivity was hampered by the extremely delicate nature of entanglement. This is where the Vienna experience made the difference.

The researchers built a giant fiber-optic Sagnac interferometer and kept the noise low and steady for several hours. This made it possible to detect enough pairs of high-quality entangled photons to surpass the rotational precision of Sagnac’s previous quantum optical interferometers by a thousand times.

In a Sagnac interferometer, two particles moving in opposite directions on a closed rotating path reach the starting point at different times. With two entangled particles, this becomes scary: they behave as a single particle testing both directions simultaneously while accumulating a delay twice as long compared to the scenario in which 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 huge coil, creating an interferometer with an effective area of ​​more than 700 square meters.

A significant obstacle the researchers faced was isolating and extracting the signal of Earth’s constant rotation. “The heart of the problem lies in establishing a reference point for our measurements, where light remains unaffected by the effect of Earth’s rotation. Given our inability to stop the Earth from rotating, we “We designed 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 turning the switch on and off, the researchers were able to effectively cancel the rotation signal at will, which also allowed them to extend the stability of their large device. “We basically tricked light into thinking it’s in a non-rotating universe,” Silvestri says.

The experiment, carried out as part of the TURIS research network hosted by the University of Vienna and the Austrian Academy of Sciences, successfully observed the effect of the Earth’s rotation on a two-photon state maximally entangled. 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 precision a thousand times higher than 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 quanta of light has finally entered the same sensitivity regimes,” says Haocun Yu, who worked about this experience as a Marie-Curie researcher. Postdoctoral fellow.

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