When high-energy light in the extreme ultraviolet or X-ray range interacts with atoms or molecules, it can cause electrons to detach from the atom. This process is called the photoelectric effect. Scientists can learn a lot about the atom being irradiated by measuring the emitted electron and its kinetic energy. This is the main idea behind photoelectron spectroscopy.
The emitted electron, called the photoelectron, is often considered a classical particle. However, it is actually a quantum object that must be described using quantum mechanics because it is so small. This means it behaves like a particle and a wave, requiring special quantum rules to describe it accurately.
Researchers at Lund University in Sweden have developed a new technique for measuring the quantum state of electrons ejected from atoms after absorbing high-energy light pulses. This breakthrough can enhance our understanding of how light interacts with matter.
David Busto, associate senior lecturer in atomic physics and one of the study’s authors, said, “By measuring the quantum state of the photoelectron, our technique can precisely address the question of ‘how quantum is the electron’. It is the same idea used in CT scans in medicine to image the brain: we reconstruct a complex 3D object by taking several 2D pictures of that object from many different angles.”
Researchers achieved this by creating the photoelectron quantum state, similar to a 3D object, through ionizing atoms with very short, high-energy light pulses. They then used a pair of laser pulses with different colors to capture 2D images, reconstructing the quantum state slice by slice.
New insights into photoelectric effect
This technique enables them to measure, for the first time, the quantum state of electrons emitted from helium and argon atoms, showing that the photoelectron quantum state varies based on the material it comes from.
Thanks to this process, researchers could fully characterize the quantum properties of the emitted photoelectrons. This technique significantly enhances the potential of photoelectron spectroscopy, specifically enabling access to previously unavailable quantum information.
Researchers tested their technique on simple atoms like helium and argon. This method could eventually be used to study molecular gases, liquids, and solids, providing insight into how ionized targets respond after losing an electron. Understanding this process could significantly impact various research fields.
Moreover, this work connects two scientific areas: attosecond science and spectroscopy with quantum information and quantum technology.
David Busto, associate senior lecturer in atomic physics, said, “This work is connected to the ongoing second quantum revolution, which aims to manipulate individual quantum objects (in this case, photoelectrons) to harness the full potential of their quantum properties for various applications.”
“Our quantum state tomography technique will not lead to the construction of new quantum computers, but by providing access to knowledge about the quantum state of the photoelectrons, it will allow physicists to exploit their quantum properties for future applications fully.”
Researchers can uncover a lot about a material’s structure by analyzing the speed and direction of the photoelectron’s emission. This is crucial for studying new materials. The technique surpasses previous methods by measuring the full quantum state of the photoelectron, providing more comprehensive information about the target than traditional photoelectron spectroscopy. This method is anticipated to help explain the processes occurring in the material after the electron is ejected.
The most astonishing part is how effectively the technique worked! Physicists had previously attempted to measure the quantum state of photoelectrons using a different method, which proved incredibly challenging. Achieving long-term stability was essential, but the team finally maintained these stable conditions.
Busto said, “The electrons emitted during the photoelectric effect contain much information about the irradiated material. By measuring the quantum state of the photoelectron, our technique can precisely address the question of “how quantum is the electron.” In the future, our technique will allow us to follow how the quantum properties of electrons evolve over time from quantum to classical.”
Journal Reference:
- Laurell, H., Luo, S., Weissenbilder, R. et al. Measuring the quantum state of photoelectrons. Nat. Photon. (2025). DOI: 10.1038/s41566-024-01607-8
Source: Tech Explorist
