Physicists Watch Atoms Spin Backward in a Crystal, a 'Textbook' First Where 1 + 1 = -1

Physicists have directly watched the rotation of atoms inside a crystal suddenly flip into reverse, capturing a counterintuitive quantum effect that the researchers believe is destined for the textbooks. The result, published in the journal Nature Physics, reveals a new rule for how angular momentum — the property that describes spinning motion — travels through solid matter.

Working with crystals of bismuth selenide, a material prized in the study of exotic quantum states, the team used ultrafast terahertz laser pulses to set the lattice of atoms vibrating. As they tracked the angular momentum moving through the crystal, they saw the atomic rotations unexpectedly switch direction midway through the process, as momentum was handed off between coupled vibrations known as phonons.

The researchers describe the phenomenon with a deliberately jarring piece of arithmetic: "1 + 1 = -1." Two angular momenta combined to produce a rotation at twice the frequency, but spinning the opposite way. The effect resembles what physicists call an Umklapp process, in which the orderly, repeating symmetry of a crystal lattice can effectively kick a wave's momentum into reverse — here driven by the crystal's rotational symmetry.

"We have discovered something fundamentally new that will hopefully make its way into the textbooks," said Sebastian Maehrlein, who led the work. The collaboration drew together scientists from the Helmholtz-Zentrum Dresden-Rossendorf and the Fritz Haber Institute of the Max Planck Society, along with collaborators across Berlin, Dresden, Jülich and Eindhoven.

Beyond its conceptual novelty, the finding could prove practically useful. The ability to steer angular momentum through a material on extraordinarily short timescales bears directly on the quest for faster, more efficient electronics. Researchers see potential applications in controlling ultrafast processes in quantum materials and in developing next-generation memory devices that store and shuttle information using the spin and rotational properties of atoms rather than electric charge alone.

The choice of bismuth selenide was no accident. The material is a so-called topological insulator, a class of crystals that conduct electricity along their surfaces while acting as insulators inside, and it has become a favorite laboratory for probing the strange ways quantum mechanics plays out in solids. The reversing rotations observed by the team are carried by chiral phonons — vibrational waves that twist in a definite handedness as they ripple through the lattice. Learning to generate and redirect those twisting vibrations on demand, the authors say, could give engineers a new lever for manipulating heat and information at the atomic scale, far faster than conventional electronics allow. For now, the most immediate payoff is conceptual: a clean demonstration that a crystal's symmetry can do something as counterintuitive as turning a spin into its own opposite.