Physicists Watch Atoms Spin Backward in Breakthrough Quantum Experiment
Scientists directly observe angular momentum moving through crystals for the first time, discovering bizarre reversal effect caused by quantum symmetry.
An international team of physicists has achieved a remarkable first: directly observing how angular momentum moves through a crystal lattice while discovering that the direction of atomic rotation can unexpectedly flip as momentum transfers between different parts of the material. The breakthrough experiment used ultra-powerful terahertz laser pulses to trigger and track tiny atomic rotations inside quantum materials, revealing fundamental insights into magnetism and quantum mechanics.
The study, led by scientists from the Helmholtz-Zentrum Dresden-Rossendorf, the Fritz Haber Institute of the Max Planck Society, and collaborators across Europe, addresses a century-old mystery about how angular momentum spreads through solid materials. While Albert Einstein and Wander Johannes de Haas demonstrated in their famous 1915 experiment that magnetic and mechanical angular momentum are linked, scientists have struggled to understand exactly how this angular momentum travels through the internal structure of crystals.
Using the material bismuth selenide, researchers employed ultra-strong terahertz laser pulses to drive one lattice vibration into circular motion, then used a second ultrafast laser to track how that rotational motion interacted with other coupled vibrations. The results revealed something extraordinary: as angular momentum moved from one vibration to another, the direction of rotation reversed. This counterintuitive effect stems from the rotational symmetry of the crystal lattice, where certain quantum states are physically equivalent even when spinning in opposite directions.
The most striking discovery was what researchers describe as a "1 + 1 = −1" quantum effect. In bismuth selenide, two angular momenta combined to produce a new rotation moving at twice the frequency but spinning in the opposite direction. This phenomenon resembles an Umklapp process, where the crystal's underlying symmetry effectively reverses motion that would normally continue in the same direction. Such processes are fundamental to understanding how energy and momentum flow through quantum materials.
The breakthrough provides scientists with their first direct view of angular momentum conservation at the atomic scale and offers new tools for controlling quantum materials. The findings could eventually lead to advances in quantum computing, spintronics, and magnetic storage technologies. By understanding how rotational motion propagates and transforms within crystals, researchers may be able to design materials with precisely controlled magnetic and quantum properties for next-generation electronic devices.
