Quantum State of Ejected Electrons Measured for the First Time
by Robert Schreiber
Berlin, Germany (SPX) Feb 18, 2025
For the first time, scientists have successfully measured the quantum state of electrons released from atoms following exposure to high-energy light pulses. This breakthrough, achieved through a novel measurement technique developed at Lund University in Sweden, promises to enhance our understanding of the interaction between light and matter.
When high-energy light, such as extreme ultraviolet or X-ray radiation, interacts with atoms or molecules, it can eject an electron in a process known as the photoelectric effect. By analyzing the kinetic energy of the released electron, researchers can glean detailed information about the atom being irradiated. This principle underpins photoelectron spectroscopy.
Traditionally, emitted electrons, or photoelectrons, have been treated as classical particles. However, in reality, these electrons exhibit quantum mechanical behavior, functioning as both particles and waves.
"By measuring the quantum state of the photoelectron, our technique can precisely address the question of 'how quantum is the electron,'" explained David Busto, an associate senior lecturer in atomic physics and co-author of the study, which has been published in Nature Photonics. "It is the same idea used in CT scans in medicine: we reconstruct a complex 3D object by taking several 2D images from different angles."
The process involves producing the quantum state of the photoelectron by ionizing atoms with ultrashort, high-energy light pulses. Two laser pulses of different colors then capture 2D images, which are compiled to reconstruct the quantum state slice by slice.
"This method has allowed us, for the first time, to measure the quantum state of electrons emitted from helium and argon atoms, demonstrating that the quantum characteristics of the photoelectron depend on the material from which it originates," said Busto.
Expanding the Scope of Photoelectron Spectroscopy
The photoelectric effect, first described by Albert Einstein over a century ago, laid the foundation for quantum mechanics. The technique of photoelectron spectroscopy, pioneered by Nobel laureate Kai Siegbahn, has been instrumental in exploring electron arrangements within atoms, molecules, and solids. Until now, however, this method relied solely on classical measurements such as electron speed. This new measurement approach allows full characterization of the quantum properties of photoelectrons, significantly expanding the analytical potential of photoelectron spectroscopy.
Applications and Future Impact
The new technique was first applied to helium and argon, which are well-understood atomic systems. However, researchers envision using it to investigate molecular gases, liquids, and solids, where quantum properties could reveal crucial details about how a material reacts after losing an electron. Such insights could influence fields like atmospheric photochemistry and the development of light-harvesting systems such as solar cells and photosynthetic processes.
Beyond its immediate applications, this work connects two scientific disciplines: attosecond science and spectroscopy (as pioneered by Nobel laureate Anne L'Huillier), and quantum information and quantum technology.
"This study aligns with the ongoing second quantum revolution, which aims to manipulate individual quantum objects to maximize their potential for future technologies," said Busto. "Our quantum state tomography technique will not lead directly to new quantum computers, but by revealing the quantum state of photoelectrons, it enables researchers to harness their quantum properties for emerging applications."
Understanding Materials at the Quantum Level
By analyzing the speed and emission direction of photoelectrons, scientists can gain deeper insights into a material's structure. This information is essential for studying new materials and understanding how they behave when exposed to light. Unlike previous techniques, this method measures the full quantum state of the photoelectron, allowing for a more detailed examination of material processes following electron ejection.
Unexpected Findings
"What surprised us most was how effectively the technique worked," Busto noted. "Previous attempts to measure the quantum state of photoelectrons using different methods proved extremely difficult due to the need for long-term stability. Achieving such stable conditions was a major breakthrough."
The Science Behind Quantum and Classical Descriptions
At the microscopic level, electrons, atoms, and molecules obey quantum mechanical principles, while larger objects adhere to classical physics. Quantum effects tend to disappear at macroscopic scales due to a phenomenon known as decoherence, where interactions between many quantum objects erase their individual quantum characteristics. Overcoming decoherence is a critical challenge in the development of quantum technologies such as quantum computing.
Electrons emitted in the photoelectric effect contain vast amounts of information about the material they originate from. Measuring the quantum state of these electrons provides a clearer understanding of their transition from quantum to classical behavior over time.
Introducing KRAKEN: A New Experimental Approach
The newly developed experimental technique, called KRAKEN, enables the precise measurement of the quantum state of photoelectrons, offering researchers an unprecedented level of insight into electron behavior and material properties.
Key Concepts: Photon, Photoelectron, and Photoelectric Effect
- Photon: A fundamental quantum of light and the electromagnetic field, not a particle in the traditional sense.
- Photoelectron: An electron ejected from an atom or molecule upon interaction with a photon.
- Photoelectric Effect: The phenomenon in which electrons are emitted from a material when exposed to electromagnetic radiation. Einstein's 1905 explanation of this effect revolutionized physics by demonstrating that light intensity does not determine electron emission-only light frequency does.
Measuring the quantum state of photoelectrons represents a major advance in our ability to study and manipulate quantum properties, paving the way for future breakthroughs in quantum mechanics and material science.
Research Report:Measuring the quantum state of photoelectrons
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