In a lab in Konstanz, Germany, physicists have created something that sounds like science fiction: electron beams that twist and flip their handedness in femtoseconds—fractions of a billionth of a second—opening a new window onto the quantum world at its smallest and fastest scales.
For decades, scientists studying matter at atomic scales have relied on combining electron microscopes with ultrafast laser technology, a pairing that has revealed how light behaves around nanoscale materials. But this approach hits a wall when researchers try to investigate rotational phenomena—the spinning of electrons, the angular momentum of atoms, or the intricate dance of particles within molecules. Peter Baum, senior author at Universität Konstanz, explains why this matters: "Whenever something happens, be it a chemical reaction, light emission, or the calculations of a modern computer, atoms and electrons must move from here to there. This happens on picometer length scales and within femtosecond or attosecond time."
To break through this barrier, Baum and his colleagues Y. Fang and J. Kuttruff devised an elegant new strategy. They transform ordinary electron waves into something extraordinary: pulses with internal torque, or twisting motion. Here's how it works: an ultrashort electron pulse is modulated by a twisted laser wave, such that different parts of the electron obtain different amounts of rotational phase. The result is an electron wave packet that can flip from left-handed to right-handed orientation within femtoseconds—a property previously achievable only in objects a million times larger.
"An ultrashort electron pulse is a short matter wave, much like the few crests and valleys of an ultrashort laser pulse, only at a million times shorter wavelength," Baum explains. Because electrons at different energies travel at different speeds, after propagation through the system, they accumulate different amounts of rotational phase. "Think of a screw that is right-handed at the tip and left-handed at the bottom," Baum says. "Our electrons still have femtosecond duration and can be focused down to atomic size."
The implications ripple across quantum science. These new electron beams could enable researchers to trigger and investigate rotational dynamics on an atomic scale—the kind of fundamental investigations that have long been out of reach. "Focusing our self-torque electron down to a single atom or a molecule and letting it scatter in an inelastic way should imprint rotational properties that might serve as a diagnostics or as a control for complex phenomena on the atomic scale," Baum notes.
The team's work, published in Nature Physics, reveals something profound: not only do these twisted electron waves solve a practical problem in quantum observation, but they also expand our understanding of what quantum objects can be. The technique is remarkably accessible, requiring only an ultrafast transmission electron microscope—the kind already present in leading research institutions. As Baum reflects, "From a fundamental perspective, it is interesting to see that objects like this can actually exist." What began as a response to experimental limitations has become a tool with potential to unlock entirely new ways of studying matter itself.