Quantum entangled photons respond to the Earth’s rotation

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

Optical Sagnac interferometers are the most sensitive devices to rotations. They have been crucial to our understanding of fundamental physics since the beginning of the last century and contributed to the development 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 these boundaries. When two or more particles are entangled, only the overall state is known, while the state of the individual particle remains undetermined until measurement. This allows more information to be obtained per measurement than would be possible without this method. 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 interferometer with optical fibers and kept the noise low and steady for several hours. This enabled the detection of enough high-quality entangled photon pairs to surpass the rotational precision of previous quantum optical Sagnac interferometers by a thousand times.

In a Sagnac interferometer, two particles traveling in opposite directions of a rotating closed path reach the starting point at different times. With two entangled particles, things get spooky: they behave like a single particle testing both directions at the same time, while building up twice the time delay compared to the no-entanglement scenario.

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

A major hurdle the researchers faced was isolating and extracting Earth’s stable rotation signal. ‘The heart of the matter lies in establishing a reference point for our measurement, where light is not affected by the Earth’s rotation effect. Given our inability to stop the Earth’s rotation, we came up with a solution: split the optical fiber into two coils of equal length and connect them via an optical switch,” explains lead author Raffaele Silvestri.

By turning the switch on and off, the researchers could effectively cancel the rotation signal as desired, also allowing them to increase the stability of their large device. “We’ve essentially tricked light into thinking it’s in a non-rotating universe,” says Silvestri.

The experiment, 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 a maximally entangled state of two photons. 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 thousandfold improvement in precision compared to previous experiments.

“That is an important milestone because, a century after the first observation of the Earth’s rotation with light, the entanglement of individual light quantas has finally reached the same sensitivity regimes,” says Haocun Yu, who worked on this experiment as a Marie-Curie. Postdoctoral researcher.

“I believe that our result and methodology will lay the foundation 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 through the curves of spacetime,” adds Philip Walther.