Experiment provides new understanding of the quantum physics of light and electrons

The study, presented in Nature Communications, was led by Imperial College London and is an international collaboration between several institutions. From Chalmers University of Technology and the University of Gothenburg, researchers Mattias Marklund and Tom Blackburn contributed to the theoretical aspects of the experiment. They developed computational models that underpin the experiment, built tools that use supercomputers to calculate the processes involved, and interpreted the experimental data.

First observation of quantum radiation reaction

The experiment was carried out at the UK’s Central Laser Facility. The research team investigated what happens when laser light and electrons travelling at almost the speed of light are directed towards each other. When they collide, the strongly charged electrons are shaken so violently by the electromagnetic field of the laser light that they begin to emit photons. In this way, the electrons lose energy and slow down. This phenomenon is known as radiation reaction, and until now its quantum-mechanical variant had not been observed directly.

“For the first time, we have been able to observe quantum properties of light and electrons in a way that has not previously been possible. We’ve also been able to determine which physical model best describes the interaction between them. The observations from the experiment give us a theoretical foundation for understanding how charged particles move in extremely strong electromagnetic fields, which is to say, in light in its various forms,” says Mattias Marklund.

A complement to particle accelerators

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The way in which the electrons slow down differs depending on whether classical physics or quantum physics is involved. In a classical scenario, the electron emits a continuous wave of light. However, if quantum mechanics governs the process, the electron emits particles of light in bursts and at random, which ultimately affects how the electrons move during the collision.

Experiments using laser light can complement those carried out at particle accelerators, as they provide an additional opportunity to test and investigate the physical laws that govern our universe. One possible future application of the experiment could be new radiation sources, such as for X-rays.

A way of bringing astrophysical environments into the lab

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This type of research is also a preliminary step towards carrying out other kinds of experiments, such as investigating what happens when light collides with light in a vacuum, or attempting to recreate astrophysical environments in the laboratory to determine how matter and antimatter are created.

“Testing our computational models helps us to understand the environments near neutron stars and black holes, where quantum properties dominate and classical physics no longer applies,” says Tom Blackburn.

Mattias Marklund adds:
“I find it exciting to see how far we can push our investigations of the laws of physics, as we did in this experiment. If we look at the room we are sitting in now, we have a table with a solid surface here and light coming in from the window there. We don’t think much about it. But in the experiment we can clearly see how light and electrons interact with each other. It’s even like a collision between lorries. These extreme parameters make it possible for physics far removed from our everyday experience to emerge. And calculating it is very, very enjoyable.”