The experiment was visualized by drawing a fiber Sagnac interferometric diagram inside the magnifying insert starting from the local position (Vienna, Austria) of the rotating Earth. Two indistinguishable photons strike the beamsplitter cube, entangle between them, and are then coupled in a fiber interferometer. Credit: Marco Di Vita
A quantum physics experiment at the University of Vienna has achieved ground-breaking accuracy in measuring the Earth’s rotation using entangled photons.
The study uses an improved optical Sagnac interferometer that uses quantum entanglement to detect rotational effects with unprecedented precision, offering potential breakthroughs in both quantum mechanics and general relativity.
Pioneering quantum experiment
A team of researchers conducted a pioneering experiment where they measured the effect of the Earth’s rotation on quantum entangled photons. The work, led by Philip Walther at the University of Vienna, has just been published in the journal Scientific advances. It represents a significant achievement that pushes the limits of spin sensitivity in entanglement-based sensors, potentially paving the way for further investigations at the intersection between quantum mechanics and general relativity.
Advances in Sagnac interferometers
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.
Sagnac interferometer built with 2-kilometer optical fibers wrapped around a 1.4-meter square aluminum frame. Credit: Raffaele Silvestri
Quantum entanglement increases sensitivity
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 on a 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. They built a huge fiber-optic Sagnac interferometer and kept the noise low and stable for several hours. This enabled the detection of sufficiently high-quality entanglement photon pairs to surpass the rotation accuracy of previous quantum-optical Sagnac interferometers by a thousand times.
Innovative techniques in quantum measurement
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.
Overcoming Challenges in Quantum Experiments
A significant hurdle the researchers faced was isolating and extracting the Earth’s steady rotation signal. “The heart of the matter,” explains lead author Raffaele Silvestri, “is to establish a reference point for our measurements where the light remains unaffected by the Earth’s rotational effect. Given our inability to stop the Earth’s rotation, we devised a solution: split the optical fiber into two coils of equal length and connect them through an optical switch.’
By turning it on and off, the researchers could effectively cancel the rotation signal at will, which also allowed them to extend the stability of their large device. “Essentially, we tricked the light into thinking it was in a non-rotating universe,” says Silvestri.
Confirmation of interactions between quantum mechanics and relativity
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 one 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 rotation sensitivity of entanglement-based sensors. This could open the way for future experiments testing the behavior of quantum entanglement across spacetime curves,” adds Philip Walther.
Reference: “Experimental observation of Earth’s rotation with quantum entanglement” by Raffaele Silvestri, Haocun Yu, Teodor Strömberg, Christopher Hilweg, Robert W. Peterson, and Philip Walther, 14 Jun 2024, Scientific advances.
DOI: 10.1126/sciadv.ado0215