A laser pulse slams into a gas cloud and knocks electrons out of it, leaving behind a bubble: a cavity almost devoid of particles. This bubble races through the plasma at nearly the speed of light, and its walls drag along the electrons trapped inside, much like a speedboat tows a water skier. Until now, such wakefield acceleration was considered a purely classical affair: particles get accelerated, but their quantum properties, such as phase twist, were not taken into account.

The authors of the study asked: what if the bubble not only accelerates the electron but also preserves its vortex twist? To find out, they solved the Dirac equation, the fundamental equation for a relativistic electron, in the field created by the plasma bubble. It turned out that the transverse motion of the electron is quantized: it can only occupy certain ring-shaped states, similar to the patterns of laser modes. The longitudinal field of the bubble acts as two potential walls between which the electron oscillates, gaining energy.

The key point is symmetry. The bubble has a shape that does not change under rotation around the axis of motion. This symmetry protects the total angular momentum of the electron: as it accelerates, it does not jump from one vortex state to another. The vortex remains clean, as long as the electron does not stray too far from the axis; the calculations give a clear criterion: the transverse displacement must be less than half the width of the wave packet.

Previously, it was unclear whether a vortex electron could be accelerated to relativistic energies without destroying its twist. Now it has been shown that it can, provided you use an axisymmetric wakefield. This turns a plasma accelerator from a simple boosting device into a moving quantum waveguide capable of carrying orbital angular momentum to high energies. Practical applications are not ready yet, but the idea opens the door to generating vortex beams for experiments in high-energy and nuclear physics.