Why Atoms Suddenly Spun Backward and What Nobody Could Explain
In a basement in Dresden, they watched a crystal do something that should be impossible. Two atoms were spinning in the same direction — angular momentum flowing between them as physics requires — and then the whole system flipped. The rotations combined into one going the opposite way. The number the researchers wrote down was 1 plus 1 equals negative 1.
The effect, documented in a paper published May 12 in the journal Nature Physics, is the first direct observation of angular momentum transfer between the vibrations that atoms make inside a solid crystal — what physicists call crystal lattice modes. For a century, scientists have understood that angular momentum is conserved: it cannot simply vanish, it must go somewhere. Einstein and Wander Johannes de Haas showed in 1915 that changing a material's magnetization makes it physically rotate. What nobody had ever seen was exactly how that angular momentum moves through the crystal lattice itself, from one vibration to another, on its way to equilibrium. Now they have.
The researchers — a ten-person team from the Fritz Haber Institute, the Helmholtz-Zentrum Dresden-Rossendorf, TU Dresden, Forschungszentrum Jülich, and Eindhoven University of Technology — used ultrashort terahertz laser pulses to drive one lattice vibration into circular motion, then watched with a second ultrafast laser as the angular momentum coupled to a second, different vibration. What they expected to see was the second vibration spin up in the same direction. Instead, it spun in the opposite direction, at twice the frequency.
The reason is the crystal's own symmetry. Bismuth selenide — a quantum material already used in photodetectors and being studied for quantum computing architectures — has a threefold rotational symmetry. Under that constraint, certain rotational states are physically equivalent even when they spin in opposite directions. When the angular momenta from two vibrations combined within that symmetry, they produced a net rotation pointing the other way. The researchers call this rotational phonon-phonon Umklapp scattering. The word Umklapp is German for flip or turnover. It is the same concept that governs how heat flows through insulators, where linear momentum is conserved in a similar symmetry-constrained way. What Minakova's team showed is that angular momentum obeys an analogous law.
"We have discovered something fundamentally new," said Sebastian Maehrlein, who runs the high-field terahertz lab at HZDR and holds a professorship at TU Dresden. His co-author Olga Minakova, then a doctoral researcher at the Fritz Haber Institute, put it more precisely: the laws of physics are directly dictated by the symmetries of nature. The experiment is the most direct experimental evidence for that claim anyone has produced.
The practical implications are farther away than the press release suggests. The technique requires expensive terahertz lasers and a carefully prepared single crystal. But the researchers argue the work establishes a new handle for ultrafast control of material properties — the ability to route angular momentum through a lattice by laser rather than by magnetic fields. If that capability scales, it becomes relevant for magnetic memory, quantum sensors, and the kind of ultrafast magnetic switching that data centers care about.
An independent News & Views commentary in Nature Physics described the result as a direct quantum mechanical signature of angular momentum conservation in solids. The paper has been on arXiv since March 2025, when the team posted their preprint before journal peer review. The fourteen-month delay between preprint and publication was not attributed to any external hold. The Radboud University researchers who wrote the News & Views contextualize the result as a contribution to ultrafast magnetism research.
A century after Einstein and de Haas asked how angular momentum moves through a solid, the answer turned out to be that it flips on its way out the door.