For the first time, scientists have watched atomic-scale spinning reverse direction inside a crystal lattice, revealing a quantum mystery that has puzzled physicists for over a century. Using intense terahertz laser pulses, an international team led by the Helmholtz-Zentrum Dresden-Rossendorf observed how angular momentum—the rotational energy at the heart of magnetism—moves through matter in ways that defy everyday intuition.
The discovery matters because it cracks open a fundamental question that has haunted physics since Einstein and Wander Johannes de Haas first demonstrated, more than a hundred years ago, that magnetism and rotation are mysteriously linked. When you change how a material is magnetized, it can actually spin. But scientists have never directly witnessed how that rotational energy travels through a solid's atomic structure—until now.
The team from HZDR, the Fritz Haber Institute of the Max Planck Society, and collaborators in Berlin, Dresden, Jülich, and Eindhoven designed an elegant experiment. They used ultra-strong terahertz laser pulses to set one lattice vibration—the coordinated motions of atoms in a crystal—spinning in circles. A second ultrafast laser then tracked what happened when that spinning motion met another vibration coupled to it nearby. What they observed was strange and elegant: as the angular momentum transferred from one vibration to the other, the direction of rotation flipped entirely.
The culprit is the crystal's own geometry. Because of the rotational symmetry built into bismuth selenide, the material they studied, certain spinning states are physically equivalent even when they rotate opposite ways. The researchers describe the effect as a kind of "1 + 1 = −1" quantum behavior—the angular momenta combined to create a new rotation spinning at double the frequency but in the opposite direction. In physics, this mirrors something called an Umklapp process, where motion gets effectively reversed because of how the crystal is structured. Though Umklapp processes are known elsewhere in condensed matter physics, this is the first time anyone has experimentally demonstrated one involving lattice angular momentum.
"I find it extraordinarily elegant how the laws of physics are directly dictated by the symmetries of nature," says Olga Minakova, the doctoral researcher at Fritz Haber Institute who served as the central experimental physicist on the project. Her mentor and study leader, Sebastian Maehrlein of HZDR and TU Dresden, captures the significance: these are "exceptionally exciting results" that may reshape how textbooks explain the quantum world.
Beyond answering a century-old riddle, the findings point toward practical power. If scientists can understand how angular momentum travels and transforms inside quantum materials, they can better control ultrafast processes—the lightning-quick interactions that govern quantum behavior. That control could unlock advances in information technologies and next-generation memory devices, giving researchers new tools to build the quantum technologies of tomorrow. The work, published in Nature Physics, suggests that the most fundamental laws governing our universe are written in the language of symmetry itself.