Distant entangled atoms acting as a single sensor provide stunning precision

Entanglement is often described as one of the most mysterious effects in quantum physics. When two quantum objects are entangled, measurements made on them can remain tightly coupled, even when the objects are far apart. These unexpected statistical connections have no explanation in classical physics. The effect can appear as if the measurement of one object somehow affects another at a distance. This phenomenon, known as the Einstein-Podolsky-Rosen paradox, has been experimentally confirmed and awarded the 2022 Nobel Prize in Physics.

Using remote entanglement for precise measurements

Based on this basis, the team led by prof. Dr. Philippe Treutlein from the University of Basel and Prof. Dr. Alice Sinatrovou of the Laboratoire Kastler Brossel (LKB) in Paris has shown that entanglement between quantum objects separated in space can serve a practical purpose. Their work shows that spatially separated but intertwined systems can be used to measure multiple physical parameters simultaneously with increased accuracy. The results of the study were recently published in the journal Science.

“Quantum metrology, which uses quantum effects to improve the measurement of physical quantities, is now an established area of ​​research,” says Treutlein. About fifteen years ago, he and his collaborators were among the first to entangle the spins of extremely cold atoms. These rotations, which can be thought of as tiny compass needles, could then be measured more precisely than if each atom behaved independently without entanglement.

“However, all these atoms were in the same place,” explains Treutlein: “We have now extended this concept by dividing the atoms into three spatially separated clouds. The result is that entanglement effects work at a distance, just like in the EPR paradox.”

Mapping Fields With Entwined Atomic Clouds

This approach is particularly useful for studying quantities that vary in space. For example, scientists interested in measuring how electromagnetic fields change from place to place can use entangled atomic spins that are physically separated. As with single-site measurements, entanglement reduces the uncertainty that arises from quantum effects. It can also cancel out perturbations that affect all atoms in the same way.

“Until now, no one has made such a quantum measurement with spatially separated entangled atomic clouds, and the theoretical framework for such measurements was also still unclear,” says Yifan Li, who worked on the experiment as a postdoctoral fellow in Treutlein’s group. Together with colleagues from LKB, the team studied how to minimize uncertainty when using entangled clouds to measure the spatial structure of the electromagnetic field.

To do this, the scientists first entangled the atomic spins in a single cloud. Then they split the cloud into three parts, which remained entangled with each other. With only a small number of measurements, they were able to determine the distribution of the field with clearly greater accuracy than would be possible without entanglement across space.

Applications in atomic clocks and gravimeters

“Our measurement protocols can be directly applied to existing precision instruments such as optical lattice clocks,” says Lex Joosten, a PhD student in the Basel group. In these clocks, the atoms are held in place by laser beams arranged in a grid and serve as extremely precise “clockwork machines”. The new methods could reduce specific errors caused by how the atoms are distributed in the lattice, leading to more accurate time measurements.

The same strategy could also improve atomic interferometers, which are used to measure Earth’s gravitational acceleration. In certain applications, known as gravimeters, scientists focus on how gravity changes in space. The use of entangled atoms allows these variations to be measured with greater precision than before.