A new route to creating order in quantum systems could be useful in developing quantum technology. Researchers at the University of Tokyo, Kazuaki Takasan and Kyogo Kawaguchi, along with Kyosuke Adachi from RIKEN—the largest comprehensive research institute in Japan—showed that raising the motility of particles results in an ordered state called ferromagnetism, sustained by repulsive forces between atoms alone.

The discovery does more than extend the concept of active matter into quantum systems, it contributes to the development of new technologies dependent on the magnetic properties of particles, like magnetic memory and quantum computing. Results were published in the journal Physical Review Research.

Flocking birds, swarming bacteria, cellular flows. These are but a few examples of the active matter state, in which individual agents—birds, bacteria, or cells—self-organize. The agents change from disordered to ordered in what is called a “phase transition.” The result is that they now move together in an organized fashion without an external controller.

“Previous studies have shown that active matter can exist at many different scales, ranging from nanometers—such as biomolecules—to even meters, such as animals,” says Takasan, first author. “However, it wasn’t known whether useful concepts in the physics of active matter could be carried over in the quantum regime. We wanted to fill in that gap.”

Flocking Birds and Ferromagnetic Order in Particles

Schematic picture of activity-induced ferromagnetism in quantum active matter. Here the moving atoms with spins show ferromagnetic order (i.e., align in one direction), similar to the flock of birds above. Credit: Takasan et al 2024

**Quantum Model Mimicking Natural Phenomena**

What the researchers had to do was plug this gap by showing one possible mechanism that might drive an ordered state into a quantum system. It was a joint work between physics and biophysics. They were inspired by the phenomenon of flocking birds, for in that case, because of the activity of every agent, the ordered state is easier to get compared to other kinds of active matter. They built a theoretical model in which atoms essentially acted like birds. Increasing the motility of atoms in this model caused the repulsive forces between atoms to reorganize them into the ordered state of ferromagnetism. Spins align in one direction in the ferromagnetic state, like birds flocking together in flight in the same direction.

“It was surprising at first to find that ordering can appear without elaborate interactions between the agents in the quantum model,” said Takasan. “It differed from expectations based on biophysical models.”

He therefore used a multi-faceted approach to ensure his finding would not be a fluke. Computer simulations, mean-field theory, statistical theory of particles, and mathematical proofs based on linear algebra agreed. This strengthened the reliability of their finding—the first step in a new line of research.

“The generalization of active matter into the quantum world has just started, and many things are still open,” says Takasan. “We would like to further develop the theory of quantum active matter and reveal its universal properties.”

This research constitutes one more major step toward understanding and harnessing the principles of quantum active matter for any potential technological applications. More generally, the notion of active matter is noticed in biological and ecological systems but now extended into the quantum regime, with implications not only for broadening an understanding of fundamental physics but also quantum computing and magnetic memory technologies of practical application.

The theoretical model by Takasan, Kawaguchi, and Adachi shows that raising the motility of particles and making good use of repulsion forces could lead to and sustain ferromagnetism in quantum systems. It is a state in which atoms’ spin is uniformly aligned, behaving ordered as it has been observed in natural phenomena such as flocking birds. Ordered states are important for applications of quantum computing where the controlled manipulation of quantum states is essential to realize complex calculations efficiently.

Their results have implications beyond theoretical physics. Demonstration by these researchers of the induction of order in quantum systems through particle motility and interaction provides a route to new materials and devices. In particular, magnetic memory technologies could greatly benefit from the development of materials that come with the robust ferromagnetic properties induced by quantum active processes.

The interdisciplinary approach the researchers have followed, from physics to biophysics, underlines the versatility and potential for applying concepts of one scientific domain into another. In this way, such cross-pollination of ideas enriches our theoretical schemes and instigates new solutions to technological problems.

In the future, the team will further explore the universal properties of quantum active matter by refining theoretical models and further experiments that will show new phenomena, and probably transformative applications in quantum technologies. Their work provides a scenario for developing quantum systems in the future to have desirable properties through controlled interactions and dynamics.

The insights from studies like this will be very important as quantum technologies evolve to shape the next generation of materials and devices. An understanding and manipulation of quantum active matter will open up new paths to applications in computing, communication, and other fields through the development of functional capabilities scientists and engineers can fully realize for a future harnessing quantum effects to enhance technological capabilities.

Their results also underscore the surprising simplicity with which order could, under certain conditions, be realized in quantum systems. When one would naively expect realizing such order to require complicated interactions, what the finding means by these researchers is that an increase in particle motility alone may suffice for realizing important organizational patterns. This challenges current paradigms in quantum physics, opening doors toward the exploration of new mechanisms of control and manipulation of quantum states.

The practical applications of ferromagnetism in quantum systems are enormous apart from the theoretical implications. Quantum computing, in particular, has so much to benefit from this ability to create and maintain ordered states of particles. Ferromagnetic materials are imperative in the encoding and processing of quantum information where the stability and predictability of operations are critical in building reliable quantum computers that can surpass the speed and efficiency of their classical counterparts.

The work of Takasan et al. exemplifies the best trend in interdisciplinary modern scientific inquiry, from physics to biophysics. By being inspired by biological systems, notably self-organized flocking behavior in birds, the authors have revealed the potential of cross-disciplinary approaches for extending our knowledge of complex phenomena within both natural and engineered systems. Such synergy enriches basic research, nurturing the very process of speeding up the translation of scientific breakthroughs into useful innovations for society.

Source: https://scitechdaily.com/

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