Physicists caught a crystal’s atoms reversing their spin for the first time

Push a crystal’s atoms into a spin one way, hand that motion to a second internal vibration, and the rotation can come out turning the other way. Physicists have now watched this happen directly inside a solid for the first time, catching the moment the lattice’s angular momentum reversed as it transferred between two of the crystal’s own vibrations.

The team describes the result with a deliberately strange piece of arithmetic: 1 + 1 = −1. Two rotations pointing the same direction combined and produced one that spun the opposite way. Nothing was actually broken in the books, because the missing twist was carried off elsewhere in the system, but the local effect is the kind of reversal that intuition does not allow.

The object in question is bismuth selenide, a crystal already prized in physics for its unusual surface behavior. What matters here is its inner clockwork. Atoms in a solid are not fixed in place; they jiggle in coordinated patterns called lattice vibrations, and some of those patterns can carry rotation, a tiny stored angular momentum that ordinarily stays politely accounted for.

To see it move, the team had to push hard and look fast. They fired ultra-strong terahertz laser pulses to drive one vibration into a circular, rotating motion, then used a second ultrafast pulse to watch what happened as that rotation coupled to a neighboring vibration. The reversal showed up in how the second pulse came back.

The interest is not the trick itself but what it opens. Angular momentum locked in vibrations is one of the hidden threads behind magnetism, and following it as it hops between vibrations gives researchers a direct handle on a process that until now had to be inferred. Control that handle, and it could become a way to steer the exotic materials that quantum technologies depend on.

The finding deserves to be read narrowly for now. It was produced in one particular crystal under laser fields far stronger than anything in everyday electronics, and the spinning that reverses is the collective rotation of the lattice, not free atoms tumbling backward like loose marbles. Whether the same reversal appears in other materials, and whether it can be harnessed rather than merely observed, are open questions.

The work, carried out by a collaboration spanning the Fritz Haber Institute of the Max Planck Society, the Helmholtz-Zentrum Dresden-Rossendorf and TU Dresden with partners in Jülich and Eindhoven, appeared in Nature Physics in May 2026. The same laser technique that revealed the reversal is the tool the groups now plan to point at other crystals, to find out how common the backward spin really is.