Physicists Observe Strange Quantum Rotation Effect That Defies Intuition

An international team of physicists has directly observed how angular momentum moves within a crystal. They used powerful terahertz laser pulses to manipulate these motions. The team found an unexpected effect: the direction of rotation flips during the transfer.

This discovery offers new insights into how magnetism works. It could also help develop better ways to control quantum materials. The study was published in Nature Physics.

Understanding Angular Momentum

Angular momentum is a fundamental property in physics. It is conserved, meaning it cannot be created or destroyed. It can only be transferred or changed into other forms. While we often think of angular momentum with spinning objects, it is also crucial in quantum physics and magnetism.

Over a century ago, Albert Einstein and Wander Johannes de Haas showed that changing a material's magnetism could cause it to rotate. This proved a strong link between magnetic and mechanical angular momentum. Since then, scientists have tried to understand how angular momentum spreads through solids. It moves by passing between the organized atoms in a crystal lattice.

Researchers from Berlin, Dresden, Jülich, and Eindhoven have now seen this process directly. Their experiments showed how angular momentum moves between different lattice vibrations. These are coordinated movements of atoms within the crystal. These findings help scientists better understand how magnetism forms in solid materials.

Lasers Reveal a Reversal

The team also showed they could control the direction of atomic motions using intense terahertz laser pulses. One laser pulse made a lattice vibration move in a circle. A second, very fast pulse then measured another connected vibration in the crystal.

During this transfer between vibrations, the researchers observed something surprising: the angular momentum reversed its direction.

This reversal happens because of the crystal lattice's rotational symmetry. Some rotational states look the same even if they spin in opposite directions. The researchers say this observation is a direct quantum mechanical sign of angular momentum conservation in solids.

A Quantum "1 + 1 = −1" Effect

The experiments focused on bismuth selenide, a quantum material. In this material, angular momentum from lattice vibrations can combine. This creates a new rotation that is twice the frequency but spins in the opposite direction.

The researchers call this unusual "1 + 1 = −1" effect an Umklapp process. Here, the crystal lattice's symmetry effectively reverses the motion's direction. This is the first time such behavior involving lattice angular momentum has been shown experimentally.

Olga Minakova, a doctoral researcher at the Fritz Haber Institute, found it "extraordinarily elegant how the laws of physics are directly dictated by the symmetries of nature."

Sebastian Maehrlein, the study leader from HZDR and TU Dresden, called these "exceptionally exciting results." He believes they have found something fundamentally new that will likely be included in textbooks.

The researchers believe this work could lead to better control of very fast processes in quantum materials. It may also help develop future information technologies and advanced memory devices.

Deep Dive & References

Observation of angular momentum transfer among crystal lattice modes - Nature Physics, 2026