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Scientists Observe Self-Organized Criticality in Quantum System

A team of researchers has experimentally visualized the concept of what is known as “self-organized criticality” in a quantum system, for the first time. The study holds implications to further develop quantum technology.

According to the concept, complex systems in a non-equilibrium state tend to develop into a critical state, that is, away from a stable equilibrium state on their own, thus reinforcing their non-equilibrium state.

At an initial glance, systems, as varied as the distribution of data on social networks or the spread of illness or fire, can have analogous properties. A case in point is an avalanche-like behavior that does not come to a standstill but rather reinforces itself. However, it is extremely difficult to study such complex systems under-regulated experimental conditions.

Now, for the first time, a research team from the European Center for Quantum Science (CESQ) based in Strasbourg, in association with scientists from the California Institute of Technology, the University of Cologne, and the University of Heidelberg, has quantitatively defined the most crucial aspects of self-organized criticality—specifically the universal avalanche behavior.

The latest study was published in the Nature journal.

The researchers initially experimented with a gas of potassium atoms, prepared by them at a low temperature, near absolute zero.

In this state, the gas is easier to control and therefore suitable to study the fundamental quantum properties of atoms.

Shannon Whitlock, Professor, Institute for Supramolecular Science and Technology, University of Strasbourg

By using lasers, the researchers activated the gas atoms, successfully influencing the interactions that occur between these atoms.

When excited, the atoms can either generate new secondary stimuli or discharge spontaneously,” elaborated Tobias Wintermantel, a Ph.D. student in the Whitlock group.

But in this example, it influenced the gas evolution in a way that interested the scientists. Upon switching on the laser, several atoms first escaped at a faster rate. But the remaining number of atoms in the gas invariably stabilized at the same value.

Another observation was made: The remaining number of particles relied on a threshold behavior and the laser’s intensity.

By comparing our experimental results with a theoretical model, we were able to determine that these two effects have the same origin.

Sebastian Diehl, Professor and Theoretical Physicist, University of Cologne

This was a first sign of the occurrence of self-organized criticality.

The experiments showed that some systems develop themselves up to their critical point of the phase transition,” added Diehl.

This was rather unexpected—during a normal phase transition, as it takes place, for instance, there is only a single critical point.

Within the boiling water, self-organized criticality would imply that the system would continue to stay floating between the gaseous phase and the liquid phase, even at the critical transition point, and even if the temperature was altered.

To date, the concept of self-organized criticality has not been validated and tested in a highly controllable physical system.

Following their experiment, the researchers returned to the lab to prove the aspect of self-organized criticality—that is, self-sustaining behavior caused by atomic decay, analogous to that of continuously replenished avalanches.

Earlier, analogous properties have only been visualized qualitatively in other contexts, for example, sunbursts or earthquakes.

For the first time, we were able to observe the key elements of self-organized criticality quantitatively and thus establish a specifically controllable atomic experiment system.

Shannon Whitlock, Professor, Institute for Supramolecular Science and Technology, University of Strasbourg

Going forward, the researchers are now planning to study how the self-organization mechanism is influenced by the quantum nature of the atoms.

We could then potentially use them in the long term to create new quantum technologies or to solve some computing problems that are difficult with conventional computers,” concluded Diehl.

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