Symmetry is one of the most fundamental principles in nature. It describes the rules that make an object look unchanged after a rotation, reflection, or other transformations. In materials, symmetry governs how atoms and electrons are arranged, and how they move together. Crucially, symmetry can even prevent certain collective atomic motions (vibrations) from interacting at all: some are simply forbidden to talk to each other. But what if those symmetry restrictions are not as rigid as they seem?

Magnons are tiny waves in magnetization that travel through solid magnetic materials, much like the ripples that spread across a pond when a stone is thrown into it. Unlike photons, which travel through empty space or optical fibers, magnons propagate within a magnetic solid. Their wavelengths can be reduced to the nanometer range, meaning that magnonic circuits could, in principle, fit onto a chip no larger than those found in today’s smartphones. Furthermore, as an excitation of a solid, a magnon naturally couples to numerous other fundamental quasi-particles—phonons, photons and others—making it an ideal building block for hybrid quantum systems and quantum metrology.