A pioneering experiment led by the University of Vienna has made it possible to measure the effect of the Earth’s rotation on quantum entangled photons.
The work, published in Science Advances, represents a significant achievement that pushes the limits of rotation sensitivity in entanglement-based sensors, which could lay the groundwork for further exploration at the intersection between quantum mechanics and general relativity.
Sagnac optical interferometers are the most sensitive devices to rotations. They have been fundamental to our understanding of fundamental physics since the early years of the last century, helping to establish Einstein’s special theory of relativity. Today, their unparalleled precision makes them the definitive tool for measuring rotational speeds, limited only by the limits of classical physics.
Interferometers that employ quantum entanglement have the potential to break those limits. If two or more particles are entangled, only the overall state is known, while the state of the individual particle remains undetermined until measurement. This can be used to obtain more information per measurement than would be possible without it. However, the promised quantum leap in sensitivity has been hampered by the extremely delicate nature of entanglement. This 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 exceed the rotational precision of previous quantum optical Sagnac interferometers by a thousand-fold.
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 ‘ghostly’: they behave like a single particle testing both directions simultaneously while accruing twice the time delay compared to the no-entanglement scenario.
This unique property is known as super-resolution. In the actual experiment, two entangled photons propagated inside a 2-kilometer-long optical fiber wound into a huge coil, resulting in an interferometer with an effective area of more than 700 square meters.
A major hurdle the researchers faced was isolating and extracting the Earth’s constant rotation signal. “The heart of the matter lies in establishing a reference point for our measurement, where the light is not affected by the Earth’s rotation effect. Given our inability to stop the Earth’s rotation, we devised an alternative solution: dividing the optical fiber into two coils of equal length and connect them using 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 apparatus. “Basically, we have tricked light into thinking it is in a non-rotating universe,” Silvestri says in a statement.
The experiment, which was carried out as part of the TURIS research network sponsored by the University of Vienna and the Austrian Academy of Sciences, has successfully observed the effect of the Earth’s rotation in a state of two-photon maximum entanglement. 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 thousand-fold better precision compared to previous experiments.
“This represents a significant milestone since, a century after the first observation of Earth’s rotation with light, single photon entanglement 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 lay the foundation for future improvements in the rotation sensitivity of entanglement-based sensors. This could pave the way for future experiments testing the behavior of quantum entanglement across space-time curves.” , adds Philip Walther.
