Szabolcs Hack, a research fellow at ELI ALPS Facility, spoke about the research.
At the heart of this research lies a question that seems simple at first glance: how does an electron move if the laser light interacting with it is described not as a classical electromagnetic wave, but as a quantum object? According to Szabolcs Hack, a research fellow in the Theoretical and Simulation Research Group at ELI ALPS Facility and an assistant professor in the Department of Theoretical Physics at the University of Szeged, the question is particularly relevant because the quantum description of strong laser fields is an area of intensive research worldwide, bridging quantum optics, strong-field physics and attosecond science.
In classical physics, the laser field is treated as an electromagnetic wave with well-defined properties. In quantum optics, however, the electromagnetic field itself is a quantum system: light can exist in various quantum states, which are inevitably accompanied by quantum uncertainties, known as quantum fluctuations. These properties do not appear in the traditional, classical description.
Following a suggestion by Péter Földi, a research fellow at ELI ALPS Facility and head of the Department of Theoretical Physics at the University of Szeged, researchers in Szeged used an analytically solvable model to investigate how these quantum properties affect a single electron wave packet. According to the results published in the New Journal of Physics, the centre of the electron wave packet largely follows the trajectory expected on the basis of classical physics. However, the effect of quantum light becomes clearly apparent in another property: the width of the wave packet. This width characterizes the quantum mechanical uncertainty of the electron’s position and, according to the calculations, is directly linked to the quantum fluctuations of the interacting laser field.
“The quantum fluctuations of the laser field manifest themselves in the quantum mechanical uncertainty of the electron’s position,” explains Szabolcs Hack.
One of the most important implications of this discovery is that the behaviour of the electron can carry information about the quantum properties of the light with which it interacts. In the longer term, this may offer new opportunities to investigate the quantum properties of light by observing the electron dynamics it induces.
Another exciting potential application of this research is that this relationship could be utilized not only for measurement but also for control. The researchers also investigated non-classical states of light, such as squeezed states, in which the quantum uncertainties of light can be controlled in a specific manner. If the quantum state of light can be controlled, the uncertainty of the electron wave packet can also be influenced through this.
In the longer term, this could even have significance in electron microscopy, where the most precise control possible over the properties of the electron wave packet plays a crucial role. The current theoretical results suggest that, with light in a suitably chosen quantum state, the spatial uncertainty of the electron wave packet can also be influenced, which may open new possibilities in the quantum control of electron beams in the future.
The research also represents a reformulation of a well-known physical problem. The interaction between a free electron and a laser field is one of the textbook examples of classical electrodynamics. The current approach, however, treats both components – the electron and the electromagnetic field – as quantum mechanical objects. The electron is described by a wave packet, while light is not simply a classical wave, but a quantized, potentially multimode electromagnetic field. This perspective provides an opportunity to reinterpret processes already well known within the classical theoretical framework, and to raise questions to which the classical description offers no answer.
The topic fits well with the research areas of quantum optics and strong-field physics, which are becoming increasingly important for ELI ALPS Facility. In addition to high-intensity, ultrashort laser pulses, growing attention is being paid to the quantum state of light. However, quantum measurements require instruments and approaches that differ, to some extent, from conventional diagnostic methods.
Therefore, the recently published results primarily constitute fundamental theoretical research, for which the researchers are planning further studies. The aim is to use an analytically solvable model to understand with increased precision the interaction between the quantized electromagnetic field and matter, and subsequently, based on the theoretical results, to develop experimental implementations and new measurement methods.
One of the most important findings of the research is that the quantum nature of light can also leave its mark on the behaviour of matter. The wave packet of a single electron can retain an imprint of the quantum state of the light with which it interacts. This could open new perspectives in the quantum diagnostics of strong laser fields and, in the longer term, contribute not only to our understanding of the interaction between quantum light and matter, but also to our ability to control it in a targeted manner.