When high-energy light with a very short wave frequency in the extreme ultraviolet or X-ray range interacts with atoms or molecules, it can cause an electron to ‘detach’ from the atom and be emitted in a process called the photoelectric effect. By measuring the light-matter interaction 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 photoelectron is a quantum object
The electron emitted, that is, what is called the photoelectron, is often treated as a classical particle. In reality, the photoelectron is a quantum object that must be described in quantum terms, as it is so small that at that scale the world is described as quantum mechanics. This means that we have to use special rules from quantum mechanics to describe the photoelectron, as it is not only a regular small particle but also behaves like a wavelength.
'By measuring the quantum state of the photoelectron, our technique can give a precise answer to the question ‘how quantum mechanical is the electron’. The idea is the same as in CT (computed tomography) scans used in medicine to image the brain: we reconstruct a complex 3D object by taking several 2D images of the object from many different angles,’ says David Busto, assistant professor of atomic physics and one of the authors of the study now published in Nature Photonics together with, among others, colleague Hugo Laurell.
First time
This is done by producing the photoelectron quantum state, which corresponds to the 3D object we want to measure, by ionising atoms with ultrashort, high-energy light pulses, and then using a pair of laser pulses of different colours to take the 2D images and reconstruct the quantum state step by step.
‘The method allows us to measure for the first time the quantum state of electrons emitted from helium and argon atoms, showing that the quantum state of the photoelectron is affected by the type of material from which it is emitted,’ says Mr Busto.
Why are these results so interesting?
'The photoelectric effect was explained more than a century ago by Einstein, laying the foundation for the development of quantum mechanics. The same phenomenon was then exploited by Kai Siegbahn to study how electrons are organised inside atoms, molecules and solids.
Paradoxically, this technique relies only 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, we finally have a method that allows us to fully characterise the quantum properties of the emitted photoelectrons, thus expanding the potential of photoelectron spectroscopy. In particular, the new measurement method 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 photoelectrons can provide a lot of information about how the ionised target reacts after the sudden loss of an electron. Understanding this process at a fundamental level may have long-term implications for various research areas. These include, for example, atmospheric photochemistry or 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 fields of science: attosecond science and spectroscopy (the kind of research that Nobel laureate Anne L'Huillier is doing) on the one hand, and quantum information and quantum technology on the other.
How can this study be important for the public?
'This work is linked to the ongoing second quantum revolution, which aims to manipulate individual quantum objects (in this case photoelectrons) to exploit the full potential of their quantum properties for various applications. Our quantum state tomography technology will not lead to the construction of new quantum computers, but by providing access to knowledge about the quantum states of photoelectrons, physicists will be able to fully exploit their quantum properties in future applications.´
What can we do with this discovery?
`By measuring the speed and direction of the photoelectron emission, we can learn a lot about the structure of the material. This is important, for example, to study the properties of new materials. Our technique allows us to go beyond previous methods by measuring the full quantum state of the photoelectron. This means we can gather more information about the target than is possible with traditional photoelectron spectroscopy. It is hoped that our technique can help to unravel the processes that take place in the material after the electron has been ejected.´
Was there anything in the results that surprised you?
`The most surprising thing is that our technique worked so well! Physicists have 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.´
The article ‘Measuring the quantum state of photoelectrons’ on the Nature Photonics website.
David Busto's profile in the Lund University research portal.