Quantum systems consisting of several particles can be used to measure
magnetic or electric fields more precisely. A young physicist at the
University of Basel has now proposed a new scheme for such measurements that
uses a particular kind of correlation between quantum particles.

In quantum information, the fictitious agents Alice and Bob are often used
to illustrate complex communication tasks. In one such process, Alice can
use entangled quantum particles such as photons to transmit or "teleport" a
quantum state—unknown even to herself—to Bob, something that is not feasible
using traditional communications.

However, it has been unclear whether the team Alice-Bob can use similar
quantum states for other things besides communication. A young physicist at
the University of Basel has now shown how particular types of quantum states
can be used to perform measurements with higher precision than quantum
physics would ordinarily allow. The results have been published in the
scientific journal Nature Communications.

### Quantum steering at a distance

Together with researchers in Great Britain and France, Dr. Matteo Fadel, who
works at the Physics Department of the University of Basel, has thought
about how high-precision measurement tasks can be tackled with the help of
so-called quantum steering.

Quantum steering describes the fact that in certain quantum states of
systems consisting of two particles, a measurement on the first particle
allows one to make more precise predictions about possible measurement
results on the second particle than quantum mechanics would allow if only
the measurement on the second particle had been made. It is just as if the
measurement on the first particle had "steered" the state of the second one.

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 it is that it works even if the particles are far
apart because they are quantum-mechanically entangled and can feel each
other at a distance. This is also what allows Alice to transmit her quantum
state to Bob in quantum teleportation.

"For quantum steering, the particles have to be entangled with each other in
a very particular fashion," Fadel explains. "We were interested in
understanding whether this could be used for making better measurements."
The measurement procedure he proposes consists of Alice's performing a
measurement on her particle and transmitting the result to Bob.

Thanks to quantum steering, Bob can then adjust his measurement apparatus
such that the measurement error on his particle is smaller than it would
have been without Alice's information. In this way, Bob can measure, for
instance, magnetic or electric fields acting on his particles with high
precision.

### Systematic study of steering-enhanced measurements

The study of Fadel and his colleagues now makes it possible to
systematically study and demonstrate the usefulness of quantum steering for
metrological applications. "The idea for this arose from an experiment we
already did in 2018 in the laboratory of Professor Philipp Treutlein at the
University of Basel," says Fadel.

"In that experiment, we were able to measure quantum steering for the first
time between two clouds containing hundreds of cold atoms each. After that,
we asked ourselves whether it might be possible to do something useful with
that." In his work, Fadel has now created a solid mathematical basis for
realizing real-life measurement applications that use quantum steering as a
resource.

"In a few simple cases, we already knew that there was a connection between
the EPR paradox and precision measurements," Treutlein says. "But now we
have a general theoretical framework, based on which we can also develop new
strategies for quantum metrology." Researchers are already working on
demonstrating Fadel's ideas experimentally. In the future, this could result
in new quantum-enhanced measurement devices.

## Reference:

Benjamin Yadin et al. Metrological complementarity reveals the
Einstein-Podolsky-Rosen paradox, Nature Communications (2021). DOI:
10.1038/s41467-021-22353-3

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