An international physics team has directly observed, for the first time, how angular momentum transfers between atomic vibrations inside a crystal — and discovered that the rotation direction flips in the process. The results, published May 12, 2026, in Nature Physics by researchers at Germany's Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and the Fritz Haber Institute of the Max Planck Society, give scientists a new experimental handle on the physics that governs magnetic states in quantum materials — and, by extension, a potential new tool for controlling the qubits at the heart of quantum computers.
The discovery closes a gap that had been open for more than a century.
Einstein's 1915 Experiment Finally Has a Crystal-Level Answer
In 1915, Albert Einstein and Wander Johannes de Haas showed that changing a material's magnetization causes it to physically rotate — a landmark experiment linking magnetic and mechanical angular momentum. Physicists have spent the decades since trying to trace exactly how angular momentum propagates through the internal architecture of solids, down to the level of individual atomic vibrations. No one had managed to watch it happen directly.
The HZDR team used intense terahertz laser pulses to drive one set of atomic vibrations — called lattice modes — into circular motion inside a crystal of bismuth selenide (Bi₂Se₃), a well-studied topological insulator with unusual quantum surface properties. A second ultrafast laser pulse then tracked the result stroboscopically, frame by frame, as that angular momentum passed into a neighboring coupled vibration.
What the researchers observed contradicted what intuition would suggest: the rotation reversed direction. Two angular momenta combining produced a result spinning the opposite way, at twice the original frequency. Lead researcher Sebastian Maehrlein, head of department at HZDR's Institute of Radiation Physics and professor at TU Dresden, called it a finding he expects to enter the physics textbooks.
"We have discovered something fundamentally new that will hopefully make its way into the textbooks," Maehrlein said, in a statement published by HZDR.
Why Atomic Spin Direction Reverses: Crystal Symmetry Dictates the Math
The reversal is not a violation of the conservation of angular momentum — it is a consequence of the crystal's own rotational symmetry. In bismuth selenide's specific lattice geometry, states that spin clockwise and counterclockwise are physically equivalent, which means a directional flip is not just permitted under the crystal's rules but dictated by them. The phenomenon is the angular momentum equivalent of a known effect called Umklapp scattering, in which a crystal's periodicity redirects rather than absorbs momentum. This is its first confirmed experimental observation in the domain of lattice angular momentum — what physicists call phonon-phonon angular momentum transfer, as documented in the published Nature Physics study.
Olga Minakova, doctoral researcher at the Fritz Haber Institute and the study's central experimental physicist, described the result as a direct demonstration of how nature's symmetries dictate observable physics. "I find it extraordinarily elegant how the laws of physics are directly dictated by the symmetries of nature," Minakova said.
Dominik Juraschek, co-author and assistant professor at Eindhoven University of Technology and Tel Aviv University, holds a European Research Council Starting Grant for the CHIRALPHONONICS research program, which focuses on novel physical phenomena arising from the angular momentum of circularly moving atoms in solids. His prior theoretical work on chiral phonons is directly confirmed at the phonon-to-phonon transfer level by this experiment.
Quantum Computing Hardware Needs Exactly This Kind of Control
The engineering significance of the finding is direct. Quantum computers rely on qubits — quantum bits that can exist in superpositions of 0 and 1. Most leading qubit architectures depend on precise control over the magnetic states of materials at the atomic scale. Angular momentum in a crystal lattice is deeply entangled with those magnetic states; altering one influences the other.
Before this experiment, no one had a direct way to observe — let alone trigger — angular momentum transfer between atomic vibrations inside a solid. The HZDR team has now shown that intense terahertz laser pulses can drive this process on demand, and that the reversal effect follows predictable symmetry rules. That combination of observability and controllability is exactly what quantum hardware engineers need to design more reliable qubits, build ultrafast quantum memory, and develop information storage that could eventually operate at less extreme temperatures than today's cryogenically cooled systems, which require environments colder than outer space to keep qubits stable.
Bismuth Selenide and Terahertz Lasers: Tools That Open a New Research Path
The material at the center of the experiment, bismuth selenide (Bi₂Se₃), is a topological insulator — a class of quantum material in which the interior behaves as an ordinary insulator while the surface carries highly mobile conducting states. Its threefold rotational symmetry is precisely what makes the angular momentum reversal not just possible but inevitable under the right excitation conditions. A 2026 review in Materials Advances identifies bismuth selenide as a leading candidate material for spintronic and quantum computing devices, citing its unique surface state properties and compatibility with next-generation device architectures.
The technique itself — driving atomic vibrations with terahertz pulses and recording the result stroboscopically — represents a repeatable, tunable method for probing the angular momentum landscape of quantum materials. Researchers say the approach opens new avenues for studying and guiding ultrafast processes in quantum materials, with potential applications in next-generation memory devices and quantum information processing. The work involved six institutions: the Fritz Haber Institute of the Max Planck Society, HZDR, TU Dresden, Forschungszentrum Jülich, Tel Aviv University, and Eindhoven University of Technology.
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Frequently Asked Questions
What is phonon angular momentum and why does it matter for quantum computing?
Phonons are quantized vibrations of atoms in a crystal lattice. When those vibrations are circular rather than back-and-forth, they carry angular momentum — a rotational quantity linked directly to magnetism. Because quantum computers rely on precise magnetic states at the atomic scale, understanding how this angular momentum moves and transforms inside a material is a prerequisite for engineering better qubit control.
How does terahertz laser physics enable scientists to observe atomic-scale rotation?
A terahertz laser pulse drives atoms in a crystal along circular orbits by resonating with specific lattice vibration frequencies. A second ultrafast probe pulse then photographs these vibrations stroboscopically — in effect, taking high-speed snapshots fast enough to capture how the rotation evolves and transfers between different vibration modes. The HZDR team used this technique to capture the phonon angular momentum reversal directly for the first time.
What is a topological insulator, and why was bismuth selenide chosen for this experiment?
A topological insulator is a quantum material that is electrically insulating in its interior but carries highly conducting states on its surface, protected by the material's symmetry. Bismuth selenide was chosen because its threefold rotational symmetry mathematically dictates the angular momentum reversal the team was looking for — making the effect both predictable and experimentally observable.
Does this quantum materials research bring quantum memory closer to room temperature?
The study does not deliver a room-temperature quantum device, but it provides the first experimental handle on a fundamental physical process — phonon-to-phonon angular momentum transfer — that governs how magnetic states in quantum materials behave at the atomic scale. Researchers describe the work as opening new directions toward ultrafast quantum memory and information storage that could eventually operate at less extreme temperatures than today's cryogenically cooled quantum hardware.
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