A group of physicists has managed to create a measurement of the Earth’s rotation using photons – particles of light – experiencing a special quantum phenomenon called entanglement. This allows them to increase the accuracy of measurements by 1000 times and could be used to investigate questions of fundamental physics.
You may be wondering why it wasn’t employed sooner. The answer is that quantum entanglement is a very delicate state. Two particles are entangled and suddenly belong to a single state. No matter how far apart they are, interacting with one will affect the other – but the state can be broken and the particles come back all unconnected.
A way to measure rotations with light is with an instrument called a Sagnac interferometer. The light is sent through the loop in opposite directions, and the rotation of the system causes one side to return to the beginning at different times. If the light in question is a pair of entangled photons going in opposite directions, something very strange happens. It’s like sending the same light in both directions at the same time and a time delay between the two twos.
To take advantage of this property, called super-resolution, researchers at the University of Vienna sent entangled photons through a 2-kilometer (1.24-mile) long optical fiber arranged in a loop. They were able to keep the system’s noise low and stable for several hours, allowing entangled photons to survive their journey through it.
Such a device is made to actively measure rotation, and there is a source of rotation under our feet. Earth. This device finally brings quantum mechanics to a level of sensitivity previously only achieved with standard unentangled light.
“This represents a significant milestone, because one hundred years after the first observation of the Earth’s rotation by light, the entanglement of individual light quanta has finally reached the same modes of sensitivity,” co-author Haocun Yu, who worked on the experiment as Marie. -Curie Postdoctoral Fellow, he said in a statement.
But actually measuring the rotation of the Earth was not the purpose of this device. The Sagnac interferometer is designed as a way to accurately measure the rotation of systems independent of the Earth’s rotation. Because of this, the team had to find a way to isolate the spin that comes from being on a rotating planet.
Sagnac interferometer in experiment. Two kilometers (1.24 miles) of optical fibers are wound around a 1.4 meter square aluminum frame.
Image credit: Raffaele Silvestri
“The heart of the matter is to establish a reference point for our measurements where light remains unaffected by the rotational effect of the Earth. Given our inability to stop [Earth] from the rotation, we devised a solution: splitting the optical fiber into two coils of equal length and connecting them via an optical switch,” explained lead author Raffaele Silvestri.
This solution, which essentially boiled down to a switch on the instrument, allowed them to cancel the Earth’s rotation signal. “Essentially, we tricked light into thinking it was in a non-rotating universe,” Silvestri continued.
The breakthrough is the first step to a new way of measuring rotation, and not only that; researchers have high expectations for possible future applications.
“I believe our result and methodology will pave the way for further improvements in the rotational sensitivity of entanglement-based sensors. This could pave the way for future experiments testing the behavior of quantum entanglement across space-time curves,” added lead author Philip Walther.
The study is published in the journal Science Advances.