For the first time, researchers have been able to measure the quantum state of electrons ejected from atoms that have absorbed high-energy light pulses. This is thanks to a new measurement technique developed by researchers at Lund University in Sweden. The results can provide a better understanding of the interaction between light and matter.

When high-energy light with a very short frequency in the extreme ultraviolet or X-ray range interacts with atoms or molecules, it can cause an electron to be “detached” from the atom and ejected in a process called the photoelectric effect. By measuring the emitted electron and its kinetic energy, a lot of information can be obtained about the atom being irradiated. This is the basic principle of photoelectron spectroscopy.

The electron that is emitted, known as the photoelectron, is often treated as a classical particle. In reality, the photoelectron is a quantum object that must be described quantum mechanically, as it is so small that at that scale the world is described in terms of quantum mechanics. This means that special rules applied in quantum mechanics have to be used to describe the photoelectron, because it is not just an ordinary small particle but also behaves like a wave.

“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,” says David Busto, associate senior lecturer in atomic physics and one of the authors of the study now published in Nature Photonics.

This is done by producing the photoelectron quantum state, which is the equivalent of the 3D object to be measured, by ionising atoms with ultrashort, high-energy light pulses, and then using a pair of laser pulses with different colours to take the 2D pictures and reconstructing the quantum state slice by slice.

“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,” says David Busto.

Why are these results so interesting?

“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.”

Paradoxically, this technique relies solely on measuring the classical properties of the photoelectron, such as its speed. Now, more than 40 years after Kai Siegbahn was awarded the Nobel Prize for photoelectron spectroscopy in 1981, there is finally a method that allows full characterisation of the quantum properties of the emitted photoelectrons, expanding the potential of photoelectron spectroscopy. In particular, the new measurement technique provides access to quantum information that would otherwise not be available.

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 ionised 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 utilise light energy, such as solar cells or photosynthesis in plants.”

Another interesting aspect of this work is that it bridges two different areas of science: attosecond science and spectroscopy (the kind of research that Nobel Prize laureate Anne L’Huillier is conducting) on the one hand, and quantum information and quantum technology on the other hand.

How might this study be important to the public?

“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.”

Was there anything in the results that surprised you?

“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.”

Key facts:
When do you choose to describe/study things quantum mechanically and not according to classical physics?

At the microscopic scale, electrons, atoms and molecules are described quantum mechanically, while on a macroscopic scale, the objects we experience in everyday life follow the laws of classical physics. Atoms and other microsystems do not behave like everyday objects. With a deliberate exaggeration, it could be said that they do not exist in the usual sense with a well-defined point and with well-defined speed. The only thing that is known is the output of the laboratories’ instruments. Since all macroscopic objects are made up of atoms and molecules that obey the laws of quantum mechanics, we might ask why we do not see quantum effects at the macroscopic scale.

In short, the reason is that when we put many quantum objects close to each other, they start to affect each other in an uncontrolled way, effectively cancelling out their individual quantum properties. This process is referred to as decoherence and is one of the key challenges that must be overcome to develop quantum technologies, such as quantum computers.

The electrons emitted during the photoelectric effect contain a lot of 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, we hope that our technique will allow us to follow how the quantum properties of electrons evolve over time from quantum to classical.

The new experimental measurement technique is called KRAKEN.

Key facts: Photon, photoelectron and photoelectric effect
A photon is not a particle in the “normal” sense. Referred to as a force-mediating elementary particle, a photon is the smallest energy quantum of light and the electromagnetic field.

A photoelectron is an electron that is ejected from an atom or molecule when it collides with a photon.

The photoelectric effect is the emission of electrons from a material when it is exposed to electromagnetic radiation (photons). Einstein’s explanation of the photoelectric effect in 1905 broke with the classical worldview of physics that the energy carried by the electromagnetic wave should be distributed evenly over the entire irradiated material. Therefore, an increase in light intensity should lead to a higher kinetic energy of the emitted electrons. However, the intensity of light does not affect this; it is the frequency of light that determines whether or not a photoelectric effect is possible. For each material, there was a cut-off frequency of the radiation that determines whether or not electron emission occurs.