Quantum measurements with entangled atomic clouds

Researchers at the University of Basel and the Laboratoire Kastler Brossel have demonstrated how quantum mechanical entanglement can be used to measure several physical parameters simultaneously with greater precision.

Entanglement is probably the most puzzling phenomenon observed in quantum systems. It causes measurements on two quantum objects, even if they are at different locations, to exhibit statistical correlations that should not exist according to classical physics – it’s almost as if a measurement on one object influences the other one at a distance. The experimental demonstration of this effect, also known as the Einstein-Podolsky-Rosen paradox, was awarded the 2022 Nobel Prize in physics.

Now, a research team led by Prof. Dr. Philipp Treutlein at the University of Basel and Prof. Dr. Alice Sinatra at the Laboratoire Kastler Brossel (LKB) in Paris has shown that the entanglement of spatially separated quantum objects can also be used to measure several physical parameters simultaneously with increased precision. The researchers recently published their results in the scientific journal Science.

Improved measurements through entanglement

“Quantum metrology, which exploits quantum effects to improve measurements of physical quantities, is by now an established field of research,” says Treutlein. Fifteen years ago, he and his collaborators were among the first to perform experiments in which the spins of extremely cold atoms were entangled with each other. The entanglement allowed them to measure the direction of the atomic spins (which can be imagined as tiny compass needles) more precisely than would have been possible with independent spins without entanglement.

“However, those atoms were all in the same location,” Treutlein explains: “We have now extended this concept by distributing the atoms into up to three spatially separated clouds. As a result, the effects of entanglement act at a distance, just as in the EPR paradox.”

The idea behind this is that if one wants to measure, for instance, the spatial distribution of an electromagnetic field, one could use an entangled state of spatially separated atomic spins. Similarly to the measurement at a single location, the entanglement could then reduce the measurement uncertainties due to quantum mechanics and, to a large degree, also cancel other disturbances that act equally on all the atomic spins.

“So far, no one has performed 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 was involved in the experiment as a postdoc in Treutlein’s group. Together with their colleagues at the LKB, Treutlein and his team investigated how the measurement uncertainty for the spatial distribution of an electromagnetic field could be minimized using such entangled clouds.

To achieve this, they first entangled the atomic spins into a single cloud and then split the cloud into three entangled parts. With only a few measurements, they were able to determine the field distribution with a distinctly better precision than would have been expected without the spatial entanglement.

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, PhD student in the Basel group. In such clocks, atoms are trapped in an optical lattice created by laser beams and then used as extremely precise “clockworks”. The methods of the Basel researchers could be used to reduce specific measurement errors arising from the distribution of atoms across the lattice, thereby improving the precision of time measurements.

Another example of a practical application is atom interferometers, which can be used to measure the Earth’s gravitational acceleration. For some applications of these instruments, known as gravimeters, the main quantity of interest is the spatial variation of gravity, which can be measured with higher precision than before using the entanglement approach.