Scientists measure a paradoxical quantum phenomenon for the first time

For the first time ever, scientists have measured an aspect of a quantum information-sharing paradox.

Scientists from TU Wien, the Max-Planck-Institut für Quantenoptik in Garching, FU Berlin, ETH Zürich and New York University have collaborated on a research study that has for the first time measured a paradoxical quantum phenomenon.

For example, imagine there is a pack of twenty dogs in front of you. If you pick one that is much longer than the pack’s average, there is a good chance that this dog also weighs more than the average. In this case, the “quantity” of this dog’s length also contains information about its weight.

Of course, this connection between different quantities can change depending on the quantities in question and the context. But according to TU Wien, quantum physics allows for stronger links between different quantities. For example, different particles or parts of a quantum system can “share” some information.

Some of the predictions surrounding the theory are quite peculiar. For instance, scientists theorise that this “mutual information” does not depend on the size of the system but only on its surface area. Now, scientists have confirmed this theory experimentally, and the results have been published in the peer-reviewed journal Nature Physics.

Understanding the quantum paradox

“Let’s imagine a gas container in which small particles fly around and behave in a very classical way like small spheres. If the system is in equilibrium, then particles in different areas of the container know nothing about each other. One can consider them completely independent of each other. Therefore, one can say that the mutual information these two particles share is zero,” said Mohammadamin Tajik of TU Wien, in a press statement.

But that is not how things work in the quantum world. If particles have quantum behaviour, you cannot quite consider them to be independent of each other. They could be mathematically connected—this means that you cannot quite meaningfully describe one particle without saying something about the other.

“For such cases, there has long been a prediction about the mutual information shared between different subsystems of a many-body quantum system. In such a quantum gas, the shared mutual information is larger than zero, and it does not depend on the size of the subsystems – but only on the outer bounding surface of the subsystem,” explained Tajik.

The experiment that proved the paradox

The international team confirmed for the first time that the “mutual information” in many-body quantum systems increases proportionally to its surface area rather than to its volume.

To test this, they conducted a study on a cloud of ultra-cold atoms. Essentially, they held particles in place with an atom chip and cooled them to just above absolute zero temperature. At the lowest temperatures, the quantum properties of particles become more significant. This is why quantum computers are run at very cold temperatures.

When systems reach such ultracold temperatures, information spreads out more and more in the system. With this, the connections between individual parts of the system become more and more important. And when this happens, such a system can be described with a quantum field theory.

“The experiment is very challenging. We need complete information about our quantum system, as best as quantum physics allows. For this, we have developed a special tomography technique. We get the information we need by perturbing the atoms just a bit and then observing the resulting dynamics. said Jörg Schmiedmayer, who led the research, in a press statement.Schmiedmayer compared this to throwing a rock into the pond and learning about the water and the pond from the ripples caused by throwing the rock.

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