A team of researchers presented a study using path-entangled quantum states of light in an interferometer, which has the ability to measure the rate of Earth's rotation.
Understanding Quantum Mechanics With Interferometers
When it comes to understanding the nature of Earth's rotation, the optical Sagnac interferometers are considered the most sensitive devices so far. They have been significant in gaining insights into fundamental physics since the early part of the last century.
Optical Sagnac interferometers did not only contribute to the establishment of Einstein's special theory of relativity. In modern times, they are also known for their unparalleled precision, which makes them the ultimate device for measuring rotational speeds, which are limited only by the confines of classical physics.
Those bounds can be broken by interferometers that employ quantum entanglement. If more than one particle is entangled, only the general state is known, while the state of the single particles remains undetermined until measured.
This possibility can be used to obtain more information per measurement than would be possible without it. However, the extremely delicate nature of entanglement limits the promised capabilities of a quantum leap in sensitivity.
Observing Earth's Rotation With Quantum Entanglement
To address this concern, a group of experts at the University of Vienna conducted a pioneering study where they measured the effect of Earth's rotation on quantum entangled photons. The details of their research is discussed in the paper "Experimental observation of Earth's rotation with quantum entanglement."
Led by Philip Walther, the team created a giant optical Sagnac interferometer while keeping the noise low and stable for several hours. This allowed the detection of enough high-quality entanglement photon pairs to outperform the rotation precision of early quantum optical Sagnac interferometers by a thousand times.
In a Sagnac interferometer, a pair of particles traveling in opposite directions of a rotating closed path reach the starting point at different times. With two entangled particles, this behavior becomes weird. They act like a single particle which tests both directions simultaneously while accumulating twice the time delay compared to the situation where there is no entanglement.
This phenomenon is known as super-resolution. In the actual study, a pair of entangled photons propagate inside a 1.2-mile (2-kilometer) long optical fiber, which is wound onto a huge coil. This gives the interferometer an effective area of over 7,535 square feet (700 square meters).
One of the challenges in this research was the isolation and extraction of the Earth's steady rotation signal. The core of the study lies in establishing a reference point for the measurement made by the authors, where the Earth's rotational effect does not affect light.
Since the team cannot stop our planet from spinning, they decided to devise a workaround. They split the optical fiber into two coils of equal lengths and connected them using an optical switch. By adjusting the switch on and off, the experts effectively canceled the rotation signal at will, enabling them to extend the stability of their large device.
The study represents an important advancement which pushes the boundaries of rotation sensitivity in entanglement-based sensors. It has the potential to push further exploration of the connection between general relativity and quantum mechanics.
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