Nele Tornow adjusts a laser in a quiet lab in Jena, Germany, where a flash lasting just a few hundred femtoseconds—less than a quadrillionth of a second—could change how we understand the invisible world of electrons. She and her team have found a new way to measure a hidden quantum feature called Berry curvature, a property that shapes how electrons move and react in materials. This tiny, elusive trait has long been predicted by theory but was nearly impossible to measure directly—until now.
Understanding how electrons behave inside materials is key to building better solar cells, faster electronics, and even quantum computers. But electrons don’t follow simple paths. Their motion is guided by quantum geometry, a kind of invisible landscape shaped by forces like Berry curvature. Think of it like a hiker navigating a mountain range: the terrain doesn’t push them, but it still shapes their path. Until recently, scientists could only map this terrain with complex math or indirect experiments. Tornow’s team has created a direct route.
Using a single atomic layer of tungsten diselenide—less than one nanometer thick, or about 100,000 times thinner than a human hair—the researchers fired two ultrashort laser pulses at the material. The first pulse, twisted like a corkscrew (circularly polarized), briefly changed the electron states. The second pulse detected the shift by measuring light emitted at double the frequency. By flipping the twist of the first laser, they saw the signal flip too—clear evidence of Berry curvature in action. From this, they calculated a precise value, matching predictions and proving their method works.
The whole process happens in a few hundred femtoseconds. That’s fast enough to catch quantum changes as they unfold. "Using this optical method, we can investigate such properties on extremely short time scales," says Giancarlo Soavi, part of the team from the University of Jena and the Abbe Centre of Photonics. This means scientists may soon watch quantum geometry shift in real time during reactions, opening doors to controlling materials with light.
Even better, the method isn’t limited to tungsten diselenide. The team, which includes researchers from the University of Regensburg, designed both the experiment and the model behind it to be widely applicable. In the future, this approach could help decode quantum geometry in many materials, leading to smarter designs for optoelectronic devices. For a field where so much is still theoretical, this is a rare and powerful step forward—one that turns a flash of light into a window on the quantum world.