Scientists measure the quantum state of electrons for the first time

The photoelectric effect, first explained by Albert Einstein in 1905, laid the foundation for quantum mechanics. It occurs when high-energy light strikes atoms, releasing electrons. This phenomenon is crucial for technologies such as spectroscopy, holography, and electron microscopy. Yet, despite its widespread use, the true quantum nature of photoelectrons has remained elusive—until now.

For the first time, researchers at Lund University in Sweden have successfully measured the quantum state of electrons emitted from atoms after absorbing high-energy light pulses. This achievement, made possible through a novel measurement technique, provides an unprecedented view into the interaction between light and matter.

David Busto, an associate senior lecturer in atomic physics and one of the study's authors, explains, “By measuring the quantum state of the photoelectron, our technique can precisely address the question of ‘how quantum is the electron.’”

Their findings, published in the journal Nature Photonics, could revolutionize multiple scientific fields, from quantum mechanics to material science, shedding light on fundamental processes that govern electron behavior.

In classical physics, an electron is often treated as a small particle with well-defined properties. However, at the quantum scale, electrons behave as both particles and waves, making their characterization far more complex. The new measurement technique developed by the researchers allows them to reconstruct the quantum state of photoelectrons with high precision.

When high-energy ultraviolet or X-ray light interacts with atoms or molecules, it causes an electron to detach in a process known as ionization. By measuring the kinetic energy and momentum of this ejected electron, scientists gain insight into the atomic structure of the material. This principle is the basis of photoelectron spectroscopy, a technique pioneered by Nobel laureate Kai Siegbahn in the 1980s.

Until now, photoelectron spectroscopy focused on measuring classical properties like electron speed and direction. But the Lund researchers have developed a method that goes further—it captures the quantum state of the electron, revealing details that were previously inaccessible.

The team’s technique uses a process similar to a CT scan. Just as multiple 2D X-ray images reconstruct a 3D representation of an organ, this new method takes snapshots of the photoelectron’s state from different angles, building a complete quantum picture. The process involves ionizing helium and argon atoms with ultrashort, high-energy light pulses, followed by an analysis using two laser pulses of different colors.

This breakthrough demonstrates that the quantum state of a photoelectron depends on the material from which it originates. Understanding these differences is crucial, as it could impact multiple scientific domains.

“The photoelectric effect was explained over a century ago by Einstein, laying the foundation for quantum mechanics. This same phenomenon was then exploited by Kai Siegbahn to study how electrons are arranged inside atoms, molecules, and solids,” Busto notes.

More than 40 years after Siegbahn’s Nobel Prize, this new method extends the potential of photoelectron spectroscopy by enabling full characterization of the quantum properties of emitted electrons. It provides access to previously unattainable quantum information, refining our understanding of atomic and molecular behavior.

Beyond fundamental physics, this discovery has far-reaching implications. The ability to measure photoelectron quantum states could lead to advancements in atmospheric photochemistry, material science, and even solar energy research.

“We applied our technique to simple atoms like helium and argon, which are relatively well understood. In the future, it could be used to study molecular gases, liquids, and solids,” Busto explains. “Understanding how the ionized target reacts after losing an electron could have a long-term impact on various fields.”

For instance, solar cells and photosynthesis rely on light-harvesting systems that convert energy at the quantum level. The new technique could help scientists better understand and optimize these systems.

This study also bridges two scientific disciplines: attosecond science and quantum information technology. It aligns with the broader goals of the ongoing second quantum revolution, which seeks to manipulate individual quantum objects for advanced applications.

While this breakthrough won’t directly lead to quantum computers, it offers physicists a powerful tool to fully exploit quantum properties in future technologies. “By measuring the speed and emission direction of the photoelectron, we can learn a lot about the structure of materials,” says Busto. “Our technique goes beyond previous methods by measuring the complete quantum state, unlocking information that traditional photoelectron spectroscopy could not access.”

The results have already surprised researchers. Measuring quantum states of photoelectrons had been attempted before, but previous methods struggled with stability issues. “The most surprising aspect is that our technique worked so well,” Busto admits. “Physicists had tried this before using different methods, and it was very challenging. Everything had to remain extremely stable for long periods, but we finally managed to achieve it.”

This stability is crucial, as the process of decoherence—the loss of quantum properties due to external interactions—poses a major hurdle in quantum research. At microscopic scales, electrons, atoms, and molecules obey quantum mechanics, while macroscopic objects follow classical physics. The transition between these two realms is still not fully understood.

As researchers refine this technique, they hope to track how electrons evolve from quantum to classical states over time. The new method, named KRAKEN, could provide key insights into decoherence, furthering the development of quantum technologies.

“The electrons emitted during the photoelectric effect contain a wealth of information about the irradiated material,” Busto says. “By measuring their quantum state, we can follow their evolution and better understand how quantum effects transition into classical physics.”

This discovery represents a leap forward in our ability to probe the quantum world. By unlocking the full quantum description of photoelectrons, scientists are not just refining existing technologies—they are expanding the boundaries of what is possible in quantum science.

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