Earth spin starry night

(Credit: Mike Laptev/Shutterstock)

VIENNA — In 1913, French physicist Georges Sagnac demonstrated that light travels at different speeds in opposite directions in a spinning frame of reference – an effect that came to be known as the Sagnac effect. Over a century later, a team of physicists at the University of Vienna has given this classic experiment a quantum twist. By injecting entangled photons into a rotating fiber optic setup, they’ve measured the rotation of the Earth itself with unprecedented precision, opening up exciting new possibilities for probing the intersection of quantum mechanics and gravity.

The team, led by Raffaele Silvestri and Philip Walther and publishing their work in Science Advances, constructed a massive quantum-enhanced Sagnac interferometer – a device that splits light into two beams that travel in opposite directions around a closed path before recombining. Due to the Earth’s rotation, the light traveling in the direction of the spin experiences a slightly shorter path than the light traveling against it, creating a detectable interference pattern when the beams remix.

But the Vienna team wasn’t content with ordinary light. They used pairs of photons that were quantum mechanically entangled, meaning their properties were inextricably linked regardless of the distance between them. When these entangled photons were sent in opposite directions around the 715 square meter fiber optic loop, the researchers observed a telltale doubling of the phase shift compared to unentangled photons – a signature of the photons’ spooky quantum connection.

Remarkably, this setup allowed the team to measure the Earth’s rotation with a sensitivity of five microradians per second — the most precise quantum optical measurement of this kind to date. That’s like being able to detect a one-degree turn of a basketball in New York City from the distance of Los Angeles.

Methodology: A Quantum Upgrade for a Classic Experiment

To achieve this, the researchers began by generating pairs of photons that were entangled in their polarization states using a process called spontaneous parametric down-conversion. These photon pairs were then sent into a Sagnac interferometer constructed from a two-kilometer long spool of optical fiber, with the two photons from each pair traveling in opposite directions.

A key innovation was the inclusion of an optical switch that could instantly change the effective area of the interferometer to zero. By toggling this switch on and off, the researchers could compare the interference patterns with and without the Earth’s rotation signal, allowing them to isolate the effect from other noise sources.

To further increase the sensitivity, the team varied the orientation of the fiber loop relative to the Earth’s rotational axis, mapping out the sinusoidal dependence of the Sagnac phase shift. This allowed them to precisely calibrate their setup and extract the maximum possible signal.

“The core of the matter lies 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,” explains lead author Raffaele Silvestri in a media release.

Sagnac interferometer built with 2-kilometers of optical fibers wrapped around 1.4 meter sided square aluminum frame.
The Sagnac interferometer was built with 2-kilometers of optical fibers wrapped around a 1.4-meter-sided square aluminum frame. (CREDIT: Raffaele Silvestri)

Results: Quantum Enhanced Precision

The results were striking. When entangled photon pairs were used, the observed phase shift was consistently twice as large as for individual unentangled photons, regardless of the interferometer’s orientation. This is a direct consequence of the quantum mechanical nature of the entangled state.

By fitting their data to the theoretical model of the Sagnac effect, the researchers determined the Earth’s rotation rate to be approximately 7.3 x 10^-5 radians per second, in excellent agreement with the accepted value. The precision achieved is orders of magnitude better than previous quantum optical measurements and begins to approach the regime where effects from general relativity come into play.

“We have basically tricked the light into thinking it’s in a non-rotating universe,” says Silvestri.

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 )

Limitations: Pushing the Boundaries

While groundbreaking, the experiment still faces some limitations. The main factor restricting the sensitivity is noise from environmental vibrations and temperature fluctuations, which can slightly change the effective size of the fiber loop. As the researchers scale up to even larger interferometers, these effects will need to be carefully controlled.

Additionally, the current setup can only measure the magnitude of the Earth’s rotation, not its direction. Future experiments could potentially use more sophisticated quantum states to gain directional information as well.

The Road Ahead: Testing Quantum Gravity

Despite these limitations, this work has profound implications. As the precision of quantum sensors continues to improve, it may soon be possible to observe subtle effects that arise from the interplay of quantum mechanics and gravity — a major unsolved problem in theoretical physics.

For instance, some theories predict that the gravitational field of a spinning mass can cause minute changes in the entanglement between particles. The techniques demonstrated in this experiment could potentially be adapted to search for such effects, providing a much-needed experimental probe of quantum gravity.

Moreover, ultra-precise quantum gyroscopes like this one could have a wide range of practical applications, from navigation of autonomous vehicles to tests of fundamental physics. By pushing the limits of quantum metrology, this research opens the door to a new era of ultra-precise sensing and tests of nature’s most elusive phenomena.

In the end, this experiment is a testament to the power of quantum entanglement and the ingenuity of experimental physicists. By applying cutting-edge quantum techniques to a century-old classic, Silvestri, Walther, and their colleagues have not only measured the spin of our planet with record-breaking accuracy but also brought us one step closer to unraveling the deep mysteries that lie at the heart of space, time, and quantum reality. As we stand on the cusp of a quantum revolution in sensing and metrology, the future looks bright – and more than a little entangled.

StudyFinds Editor-in-Chief Steve Fink contributed to this report.

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