Imagine a world where the fundamental laws of physics, like Newton's third law of motion, can be bent or even broken. Sounds like science fiction, right? But here's where it gets controversial: researchers from Japan have discovered a way to do just that—using light. Their groundbreaking work reveals how photoinduced non-reciprocal magnetism can effectively violate Newton's third law, opening up a new frontier in materials science. And this is the part most people miss: this isn't just about breaking rules; it's about harnessing this phenomenon for revolutionary applications in light-controlled quantum materials.
In the world of physics, equilibrium systems follow the principle of action and reaction, minimizing free energy. However, non-equilibrium systems, such as biological or active matter, often exhibit non-reciprocal interactions—where the response isn't equal or opposite to the action. Think of the brain's neurons, predator-prey relationships, or colloids in optically active media; these all showcase non-reciprocity. But can we replicate this in solid-state systems? A team led by Associate Professor Ryo Hanai from the Institute of Science Tokyo, alongside collaborators from Okayama and Kyoto Universities, says yes. They've developed a theoretical framework to induce these interactions in solids using light, published in Nature Communications (DOI: 10.1038/s41467-025-62707-9).
"We've found a way to transform ordinary reciprocal spin interactions into non-reciprocal ones using light," Hanai explains. For instance, the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction in magnetic metals can become non-reciprocal when exposed to light at a specific frequency. This frequency selectively opens decay channels for certain spins, creating an energy imbalance that drives non-reciprocal behavior.
The team's dissipation-engineering scheme leverages light to activate these decay channels, leading to a fascinating phenomenon: a non-reciprocal phase transition. In a bilayer ferromagnetic system, one layer tries to align with the other, while the other resists, resulting in a spontaneous, persistent 'chase-and-run' rotation of magnetization. This 'chiral' phase is a direct consequence of broken action-reaction symmetry, and remarkably, the light intensity required is within reach of current technology.
Here’s the bold part: This isn’t just a theoretical curiosity. It bridges active matter and condensed matter physics, with potential applications in Mott insulating phases, multi-band superconductivity, and even new spintronic devices. But it also raises questions: How far can we push the boundaries of non-reciprocity? Could this lead to technologies we haven’t even imagined yet?
This research not only challenges our understanding of physical laws but also paves the way for next-generation technologies. What do you think? Is this a step toward a new era in materials science, or are we treading into uncharted and potentially risky territory? Let’s discuss in the comments!
