Controlling Electron Motion Opens The Door To Next-Generation Of Quantum Technologies

A team of physicists from Czech research institutions shows how shaped light fields can steer ultrafast processes inside solids with remarkable precision

Researchers from the Division of Optics of the Institute of Physics of the Czech Academy of Sciences, the Faculty of Mathematics and Physics at Charles University, and the IT4Innovations National Supercomputing Center at VSB – Technical University of Ostrava have succeeded in controlling the motion of electrons in solids with exceptional precision — on the attosecond timescale, i.e., in intervals of a billionth of a billionth of a second (10^⁻¹⁸ s). Their findings have been published in the prestigious journal Physical Review Letters as editor’s suggestion.

This control was achieved by combining two laser beams of different frequencies, which together form a light field with a finely tunable waveform. These shaped pulses allow researchers to determine the exact instant an electron transitions into the conduction bands — in other words, the moment it is freed and begins moving through the material.

When Light Waves Work Together
Imagine a bead moving smoothly along a perfect sine wave. Now combine that wave with another one that oscillates three times faster. The result is a new, asymmetrical waveform — higher in some places, lower in others — whose exact shape depends on the relative timing between the two original waves. It’s this resulting waveform that enables scientists to precisely control the timing of electron release.

And if the electron is released at just the right moment, it later recombines with a “hole” thanks to the oscillating light field. Upon recombination, it emits excess energy in the form of a photon — one with much higher energy than the laser that initiated the process. This phenomenon is known as High Harmonic Generation (HHG).

“By precisely controlling the timing of electron release, we can study ultrafast processes in a precision that was previously inaccessible to modest laboratories, but mostly to large research facilities. It’s fascinating how sensitively the electron responds to even subtle changes in the shape of the light field, and how precise can be our predictions for this,” said Thibault J.-Y. Derrien, co-author of the study and group leader in “Ultrafast Photonics”, part of the department of “Scientific Laser Applications.”

What the Scientists Were Investigating
The aim of the research was to benchmark the readiness of prediction capabilities of quantum simulation codes that are available nowadays. High harmonic generation spectroscopy is an extremely precise observable that enables to access to the behaviour of charge dynamics in solids during the laser irradiation. The team studied how electrons behave in crystalline silicon when irradiated by two overlapping laser pulses — one at a fundamental infrared frequency and the other at its third harmonic. The pulses were precisely synchronized to form a light field with a controlled temporal structure.

The researchers found that the shape of the laser field affects not only how much light is emitted, but also the exact timing of that emission. Moreover, they demonstrated that this timing can be finely tuned by adjusting the relative phase between the two laser pulses within a tabletop setup. Interestingly, the moment electrons start moving through the material does not coincide with the peak of high-frequency light emission. Even a tiny addition of third-harmonic light — as little as 5% of the intensity — can significantly shift the outcome.

A Gateway to the Ultrafast World
The ability to control electron motion with attosecond precision opens up new avenues — from developing ultrafast electronic and photonic devices, to exploring quantum processes in advanced materials, and even to creating novel diagnostic tools for studying the structure of solids. The method acts like a kind of “time-resolved X-ray”, revealing what happens inside a material during its interaction with light.

“I am truly delighted by the great success achieved by our colleagues from the Department of Scientific Laser Applications. The team only joined the Division of Optics this January, and they are already delivering such remarkable results. This is an excellent example of combining expertise with effective collaboration between teams across Czech scientific institutions.” said Alexandr Dejneka, head of the Division of Optics at the Institute of Physics.