Quantum entanglement reacts to Earth’s spin

Quantum entanglement reacts to Earth's spin.

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A groundbreaking experiment led by Philip Walther and his team at the University of Vienna has recently been published in Science Advances. The study measured the impact of the Earth‘s rotation on quantum entangled photons, demonstrating a significant advancement in rotation sensitivity for entanglement-based sensors. This achievement has the potential to pave the way for further exploration at the intersection of quantum mechanics and general relativity.

Optical Sagnac interferometers are the most sensitive devices for measuring rotations and have played a vital role in advancing our understanding of fundamental physics. Since the early 20th century, these devices have contributed to the establishment of Einstein’s special theory of relativity.

Today, their unparalleled precision makes them the ultimate tool for measuring rotational speeds, pushing the boundaries of classical physics.

Interferometers utilizing quantum entanglement hold the key to exceeding current limitations. When particles are entangled, the individual particle states remain undetermined until measurement, allowing for more information per measurement. However, realizing this potential has been challenging due to the delicate nature of entanglement.

The Vienna experiment overcame this obstacle by constructing a large optical fiber Sagnac interferometer with stable, low noise levels for extended periods. This breakthrough enabled the detection of high-quality entangled photon pairs, surpassing the precision of previous quantum optical Sagnac interferometers by a thousandfold.

The experiment was pictured drawing a fiber Sagnac interferometric scheme inside a magnifying inset starting from a local position (Vienna, Austria) of the rotating Earth. Two indistinguishable photons are incident on a beam splitter cube, entanglement between them is created, and then they are coupled in the fiber interferometer.
The experiment was pictured drawing a fiber Sagnac interferometric scheme inside a magnifying inset starting from a local position (Vienna, Austria) of the rotating Earth. Two indistinguishable photons are incident on a beam splitter cube, entanglement between them is created, and then they are coupled in the fiber interferometer. Credit: Marco Di Vita

In a Sagnac interferometer, the behavior of entangled particles is truly fascinating. When two particles travel in opposite directions on a rotating closed path, they reach the starting point at different times.

However, when the particles are entangled, they seem to exhibit a mysterious property where they act as a single particle exploring both directions simultaneously, resulting in a time delay twice as long as when they are not entangled. This phenomenon, known as super-resolution, has significant implications.

In a recent experiment, researchers utilized a 2-kilometer-long optical fiber wound around a large coil to create an effective interferometer area of over 700 square meters for two entangled photons. This setup allowed for the exploration of the unique properties of entangled particles in the context of Sagnac interferometry.

Despite the fascinating prospects, isolating and extracting Earth’s steady rotation signal presented a major challenge for the researchers.

“The core of the matter,” explains lead author Raffaele Silvestri, “lays in establishing a reference point for our measurement, where light remains unaffected by Earth’s rotational effect. Given our inability to halt Earth from spinning, we devised a workaround: splitting the optical fiber into two equal-length coils and connecting them via an optical switch”. By toggling 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. “We have basically tricked the light into thinking it’s in a non-rotating universe,” says Silvestri.

The experiment conducted as part of the research network TURIS hosted by the University of Vienna and the Austrian Academy of Sciences has successfully observed the effect of the rotation of Earth on a maximally entangled two-photon state, confirming the interaction between rotating reference systems and quantum entanglement. The result provides a thousand-fold precision improvement compared to previous experiments and aligns with Einstein’s special theory of relativity and quantum mechanics.

“That represents a significant milestone since, a century after the first observation of Earth’s rotation with light, the entanglement of individual quanta of light has finally entered the same sensitivity regimes,” says Haocun Yu, who worked on this experiment as a Marie-Curie Postdoctoral Fellow. “I believe our result and methodology will set the ground 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 through the curves of spacetime”, adds Philip Walther.

Journal reference:

  1. Raffaele Silvestri, Haocun Yu, Teodor Strömberg, Christopher Hilweg, Robert W. Peterson, Philip Walther. Experimental observation of Earth’s rotation with quantum entanglement. Science Advances, 2024; DOI: 10.1126/sciadv.ado0215

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