With quantum systems, which consist of several particles, magnetic or electric fields can be measured more precisely. A young physicist at the University of Basel has now proposed a new method for such measurements that is based on a certain type of correlation between quantum particles.

In quantum informatics, the fictitious agents Alice and Bob are often used to make complex communication tasks clear. For example, Alice can use entangled quantum particles such as photons to transmit a – unknown to herself – quantum state to Bob, that is to “teleport” it, which is not possible with traditional communication.

So far, however, it has not been clear whether the Alice-Bob team can use similar quantum states for other things besides communication. A young physicist at the University of Basel has now shown how measurements can be made with certain types of quantum states with a higher precision than quantum mechanics normally allows. The results were published in the journal Nature Communications.

#### Quantum control at a distance

Together with researchers in Great Britain and France, Dr. Matteo Fadel from the Department of Physics at the University of Basel is thinking about how to approach highly precise measurement tasks using what is known as “quantum steering”.

Quantum control describes the fact that in certain quantum states of two particles a measurement on the first particle allows more accurate predictions to be made about possible measurement results on the second particle than quantum mechanics would allow for a single measurement on the second particle. This is just as if the measurement on the first particle had “controlled” the state of the second.

This phenomenon is also known as the EPR Paradox, named after Albert Einstein, Boris Podolsky, and Nathan Rosen, who first described it in 1935. What is remarkable about this control is that it also works when the particles are far away from each other, since they are entangled quantum mechanically and sense each other at a distance. This also allows Alice to send her quantum state to Bob through quantum teleportation.

“For quantum control, the particles have to be entangled with one another in a very specific way,” explains Fadel. “We now wanted to understand whether this could also be used to make better measurements”. The measurement procedure that he proposes is for Alice to take a measurement on her particle and send the result to Bob.

Thanks to the quantum control between the particles, he can then adjust his measuring apparatus so that the measurement error on his particle is smaller than it would have been without Alice’s information. In this way, Bob can, for example, measure magnetic or electric fields that act on his particle with high precision.

### Systematic investigation of measurements with quantum control

The work of Fadel and his colleagues now makes it possible to systematically investigate and demonstrate the usefulness of quantum control for metrological applications. “The idea for this arose from an experiment that we carried out in 2018 in Professor Philipp Treutlein’s laboratory at the University of Basel,” says Fadel

“At that time, we were able to measure the quantum control between two clouds of hundreds of cold atoms for the first time. Then we asked ourselves whether something could be done with it. ” With his work, Fadel has now created a mathematically solid foundation for the implementation of realistic measurement applications that use quantum control as a resource.

“In some simple cases there was already a connection between the EPR paradox and precision measurements,” says Prof. Philipp Treutlein, “but now we have a general theoretical framework within which we can also develop new strategies for quantum metrology.” Some researchers are already working on demonstrating Fadel’s ideas in experiments. In the future, this could lead to new quantum-amplified measuring devices.

**Featured image:** Two interconnected particles*Einstein-Podolski-Rosen correlations can be used for precision measurements. (Image: Jurik Peter, Shutterstock)*

*Original publication*

*Benjamin Yadin, Matteo Fadel, and Manuel Gessner**Metrological complementarity reveals the Einstein-Podolsky-Rosen paradox**Nature Communications (2021), doi: 10.1038 / s41467-021-22353-3*

*Provided by **University of Basel*