Researchers at the University of Basel have uncovered a fascinating phenomenon in quantum physics: antagonistic interactions within quantum systems. These are interactions where one particle attracts another, but in return, the second particle repels the first. This discovery challenges the conventional understanding of physics, where, under most conditions, particles either attract or repel each other equally in a mutual, reciprocal fashion. The study, published in the journal Physical Review X, reveals the possibility of this unusual form of interaction occurring in the quantum world, offering new avenues for practical applications and experimental setups.
Intuitive vs. Antagonistic Interactions: A Difference in Dynamics
We’re all familiar with classical physics principles, particularly in terms of electrostatic forces. For example, charges of opposite polarity naturally attract each other, and like charges repel. This behavior is both intuitive and predictable: when charge A attracts charge B, charge B reciprocates by attracting charge A. It’s a straightforward interaction based on symmetry—a concept engrained in our basic understanding of the physical world.
But nature doesn’t always follow these simple, symmetrical rules. One striking example of this occurs in the natural world, notably in predator-prey dynamics. Consider the relationship between a fox and a rabbit. The fox is attracted to the rabbit, seeking it out as prey, while the rabbit, instead of being attracted, naturally recoils and runs away. The fox chases, while the rabbit attempts to escape. Here, there’s no mutual attraction. Instead, the fox’s behavior draws it toward the rabbit, but the rabbit’s actions oppose this draw by repelling the predator.
In systems beyond biology, this kind of dynamic interplay is also seen in other fields, including nanoparticles, colloids, and systems involving active agents, where particles are dispersed and exert opposing forces on each other. The researchers from Basel wanted to investigate whether something akin to this dynamic could be present in quantum systems.
Quantum Mechanics: New Discoveries and Approaches
In a typical quantum system, particles interact in ways that are often mutually beneficial or reciprocal, where force or energy is transferred between them in a consistent, bilateral manner. This “two-way street” behavior, often found in closed quantum systems, seemed to undermine the possibility of antagonistic, nonreciprocal interactions. But physicists Tobias Nadolny, Prof. Christoph Bruder, and Dr. Matteo Brunelli sought to determine if it could indeed be possible for quantum systems to exhibit a “fox-chasing-rabbit” dynamic.
One of the key difficulties they faced was creating an antagonistic interaction. In most classical physics experiments and quantum systems, particles exist in a closed system, exchanging energy and force in predictable ways. However, the researchers hypothesized that in an open quantum system, energy continuously flows in and out of the system, which could allow for a form of “active” behavior in the particles. In this scenario, particles could be “pushed” or influenced by external sources, such as light, and interact in non-reciprocal ways.
Through meticulous theoretical work, they confirmed that such behavior was indeed possible. The key to realizing antagonistic interactions in quantum systems lies in creating a situation where external energy continuously supplies the particles—making them “active” rather than just passive responders to forces.
Open Quantum Systems and Antagonism
An open quantum system refers to a system that interacts with its environment or outside sources of energy. When energy, such as light or other fields, interacts with quantum particles, it alters their properties and behavior, creating what are called “non-equilibrium” conditions. In this case, the particles, energized by these external forces, become more dynamic or “active.”
Dr. Matteo Brunelli, a postdoctoral researcher on the project, mentions, “Initially we had no idea whether this was going to work at all… but after extensive calculations, the result was clear: quantum particles can indeed exhibit antagonistic interactions, akin to predator-prey dynamics.” This “active” behavior facilitated the possibility of one group of quantum particles attracting another, while the second group pushed back against the first.
Interestingly, this behavior results in dynamic, non-static interactions within the quantum system. The researchers suggest that this kind of relationship prevents the system from reaching a steady equilibrium. Instead, the quantum particles stay in constant motion and interaction, never resting or achieving static states.
The concept of time crystals—an unusual state of matter in which the system’s internal state perpetually repeats in time—emerges as a natural consequence of such dynamic, antagonistic interactions. In time crystals, there is no outside force dictating regular motion; rather, the system perpetuates its own internal rhythm, leading to motion that never ceases. This concept is radically different from conventional crystal structures, where regularity and symmetry emerge in space. Instead, time crystals achieve periodic repetition within time itself.
Realizing Antagonistic Interactions with Coupled Atoms
The next key challenge for the researchers was to identify a practical system in which antagonistic interactions could occur. This involved carefully structuring an open quantum system with a specific set of conditions that would enable antagonistic behavior. Their theoretical blueprint suggests using cold atoms—atoms cooled to extremely low temperatures to control their quantum state—and coupling them together via waveguides.
Waveguides, such as optical fibers, allow light to propagate in controlled ways. These waveguides could be strategically positioned in such a manner that light is constrained to travel from right to left in one waveguide and from left to right in the other, forming a controlled pathway for quantum energy to pass. When two distinct groups of cold atoms are coupled through these waveguides, their quantum properties—specifically their spin phases—become coupled as well. Spin refers to the intrinsic angular momentum of particles, which can be visualized as tiny rotating arrows.
In the quantum system described by the researchers, the spins in group A of atoms want to align with those of group B, while group B’s spins will attempt to repel the spins of group A, seeking maximal difference. This sets up a situation akin to a mutual antagonism: Group A’s attraction to Group B’s state contrasts with B’s attempt to create disalignment.
The result is that these quantum spins behave in opposition, leading to the desired antagonistic quantum interaction. If successful, this behavior would enable particles to engage in consistent, dynamic movements—akin to predator-prey interactions—but in the quantum realm.
Potential Applications of Antagonistic Quantum Interactions
While the theoretical and experimental details of this quantum discovery are still being explored, the implications for practical applications are profound. These results open doors for creating more dynamic quantum systems that maintain constant motion and interaction without reaching an equilibrium state. The team hopes that future research will focus on implementing these systems for experimental use.
Nadolny, one of the authors of the study, adds, “We hope our work will inspire other researchers to investigate quantum systems with antagonistic interactions further.” Notably, the understanding of antagonistic quantum systems could have significant implications in several fields, one of which is the development of ultra-precise quantum technologies, such as atomic clocks.
For instance, in atomic clocks, a central challenge is achieving extreme accuracy by minimizing errors introduced by the quantum system’s dynamics. The constant motion and dynamic qualities seen in antagonistic quantum systems could lead to more stable and robust atomic clockworks, resulting in even more accurate time measurement and improvements in fields like GPS technology or quantum computing.
Conclusion
The groundbreaking research by the team at the University of Basel has introduced a fascinating concept to the world of quantum mechanics: the idea of antagonistic quantum interactions. The study reveals that quantum particles can indeed behave like foxes and rabbits, engaging in attraction and repulsion in a way that has never been observed before. By using open quantum systems with active particles, their work presents an entirely new avenue of research that promises to expand our understanding of quantum behavior and its real-world applications.
As physicists continue to explore the implications of these findings, it’s clear that the discovery of non-reciprocal interactions in quantum mechanics may lead to revolutionary advances, particularly in the fields of quantum technologies and precision measurement. With more experiments to be conducted and insights to be gained, the future of quantum physics is poised for further surprises and groundbreaking developments.
Reference: Tobias Nadolny et al, Nonreciprocal Synchronization of Active Quantum Spins, Physical Review X (2025). DOI: 10.1103/PhysRevX.15.011010