For the first time, researchers have successfully measured the quantum state of electrons ejected from atoms that have absorbed high-energy light pulses, thanks to a novel measurement technique developed by researchers at Lund University, Sweden. This advancement enhances the understanding of light-matter interactions. The study was conducted by David Busto, an associate senior lecturer in atomic physics, along with his team and published in Nature Photonics.
Image Credit: Lund University
When high-energy light, particularly in the extreme ultraviolet or X-ray range, interacts with atoms or molecules, it can lead to the ejection of an electron from the atom, a phenomenon known as the photoelectric effect. By analyzing the emitted electron and its kinetic energy, valuable insights can be gained into the irradiated atom. This principle underlies photoelectron spectroscopy.
Typically, the emitted electron, referred to as the photoelectron, is regarded as a classical particle. However, it is fundamentally a quantum object that requires a quantum mechanical description due to its minuscule size. Consequently, the special rules of quantum mechanics must be applied to characterize the photoelectron, as it exhibits both particle-like and wave-like properties.
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 used in medicine to image the brain: we reconstruct a complex 3D object by taking several 2D pictures of that object from many different angles.
David Busto, Study Author and Associate Senior Lecturer, Lund University
This measurement is achieved by generating the photoelectron quantum state, akin to a 3D object, through the ionization of atoms using ultrashort, high-energy light pulses. Subsequently, a pair of laser pulses of different colors is employed to capture 2D images, allowing for the reconstruction of the quantum state in slices.
“The technique allows us to measure for the first time the quantum state of electrons emitted from helium and argon atoms, demonstrating that the photoelectron quantum state depends on the type of material from which it is emitted,” added David Busto.
Why are These Results So Interesting?
Busto noted, “The photoelectric effect was explained over a century ago by Einstein, laying the foundations for the development of quantum mechanics. This same phenomenon was then exploited by Kai Siegbahn to study how electrons are arranged inside atoms, molecules, and solids.”
Interestingly, this technique relies solely on classical measurements of the photoelectron, such as its velocity. More than 40 years after Kai Siegbahn received the Nobel Prize for photoelectron spectroscopy in 1981, a method has emerged that enables comprehensive characterization of the quantum properties of emitted photoelectrons, thereby broadening the scope of photoelectron spectroscopy. This new measurement technique grants access to quantum information that was previously unattainable.
How Can These Results Be Useful?
“We applied our technique to simple atoms, helium, and argon, which are relatively well known. In the future, it could be used to study molecular gases, liquids, and solids, where the quantum properties of the photoelectrons can provide a lot of information about how the ionized target reacts after the sudden loss of an electron. Understanding this process at the fundamental level could have a long-term impact on various fields of research. Examples include atmospheric photochemistry or in the study of light-harvesting systems, which are systems that collect and utilize light energy, such as solar cells or photosynthesis in plants,” Busto stated.
Another fascinating aspect of this research is its link between two scientific fields: attosecond science and spectroscopy—an area being explored by Nobel Prize laureate Anne L’Huillier—and quantum information and technology.
How Might This Study Be Important to the Public?
Busto further added, “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 fully exploit their quantum properties for future applications.”
What Can the Discovery Be Used for?
“By measuring the speed and emission direction of the photoelectron, we can learn a lot about the structure of the material. This is essential, for example, to study the properties of new materials. Our technique allows us to go beyond previous methods by measuring the complete quantum state of the photoelectron. This means that we can gather more information about the target than what is possible with traditional photoelectron spectroscopy. It is hoped that our technique can help unravel the processes that occur in the material after the electron has been ejected,” Busto stated.
Was There Anything in the Results That Surprised?
Busto noted, “The most surprising aspect is that our technique worked so well! Physicists had already tried to measure the quantum state of photoelectrons using a different method, and those experiments showed that it is very difficult. Everything has to be very stable over a long period of time, but we finally managed to achieve these very stable conditions.”
When do Researchers Choose to Describe/Study Things Quantum Mechanically and Not According to Classical Physics?
On a macroscopic level, the things people encounter in daily life adhere to the principles of classical physics, but electrons, atoms, and molecules are described by quantum mechanics at the microscopic level. The behavior of atoms and other microsystems differs from that of ordinary objects. You could argue that they don’t exist in the traditional sense, with a clearly defined position and pace.
The only certainty is the data recorded by the laboratory instruments. Since all macroscopic objects consist of atoms and molecules that follow the laws of quantum mechanics, it raises the question: why don’t we observe quantum effects on a macroscopic scale?
To put it succinctly, the reason is that when numerous quantum objects are placed near one another, their effects become uncontrollable and effectively cancel out each other's quantum characteristics. One of the main obstacles to the development of quantum technology, including quantum computers, is this phenomenon, which is known as decoherence.
Many details about the irradiated material can be found in the electrons released during the photoelectric effect. The method can accurately answer the query "How quantum is the electron" by measuring the photoelectron's quantum state. Researchers anticipate that their method will enable them to track the evolution of electrons' quantum properties from quantum to classical throughout time.
The new experimental measurement method is called KRAKEN.
Journal Reference:
Laurell, H. et. al. (2025) Measuring the quantum state of photoelectrons. Nature Photonics. doi.org/10.1038/s41566-024-01607-8