New experiment reveals quantum behaviour in powerful laser-electron collisions

An Imperial led study has provided the first clear evidence of quantum radiation reaction, reshaping our understanding of strong-field physics.

In a groundbreaking study, researchers in Imperial’s Department of Physics have observed significant energy loss in high-energy electrons when they collide with intense laser pulses.

Their research, released in Nature Communications, provides the first clear experimental evidence of quantum radiation reaction in strong electromagnetic fields, marking an important step toward understanding how the laws of physics operate in some of the most extreme environments in the universe.

The experiment at the UK’s Central Laser Facility was led by Imperial College London, with collaboration from institutions such as the University of York and Queen’s University Belfast.

A quantum kick

When high-energy electrons – electrons that have been accelerated to near the speed of light carrying hundreds of millions of electronvolts of energy - collide with an ultra-intense laser pulse, they lose energy.

In these collisions, the electrons are shaken so violently by the laser’s electromagnetic fields that they emit radiation – in the form of high-energy photons – and recoil, much like a gun kicking back when it fires a bullet. It is this “kick-back” force, known as radiation reaction, that causes the electron to lose energy.

Radiation reaction can arise whenever charged particles encounter electric or magnetic fields and is well described by classical physics in weak fields. However, in extreme environments like those near black holes and neutron stars, classical physics breaks down and quantum effects dominate. In such environments, a quantum theory of radiation reaction is needed to accurately describe the behaviour of light and matter.

Until now, quantum radiation reaction has not been observed directly, because recreating these conditions in a laboratory setting is challenging. Current laboratory studies require electrons to move almost at the speed of light before colliding with a highly powerful laser. Furthermore, the electrons and laser must be aligned with extraordinary precision.

This lack of experimental tests has left a critical gap in our understanding of how light and matter behave in these environments.  

Through their research, the team has closed that gap, observing this ‘kick-back’ in strong fields for the first time. Their results reveal such severe energy losses that only a quantum theory of light and matter in strong fields can explain them.

Inside the experiment

The experiment, conducted on the Gemini Laser at the Rutherford Appleton Laboratory, used two high powered lasers. The first laser accelerated electrons through laser-driven wakefield acceleration, a process where a laser pulse creates a plasma wave that electrons can ‘surf’ to rapidly gain energy.

The second laser then collided head-on with these energetic electrons. To replicate the strong electromagnetic fields found near cosmic phenomena, this laser was incredibly powerful, with a peak power equivalent to 500,000 power stations, but delivered over an ultra-short timescale.

Achieving successful collisions, where electrons experienced the full intensity of the laser’s electromagnetic fields, required the laser and electrons to be overlapped with extraordinary precision: within a millionth of a meter in space and to roughly a millionth of a millionth of a millisecond in time.

The first author of the study, Eva Los, then a PhD student at Imperial College London, said “When the electrons collide with the laser, they are accelerated so strongly that they emit gamma rays.  To emit these gamma rays, the electrons had to lose about as much energy as they would if they had passed through about a millimetre of lead. The gamma-rays produced are so energetic that you’d need a metre of concrete to stop one of them.

“We have seen that in these extreme fields, a quantum theory is needed to describe how light and particles behave. This result has been many years in the making and is a really exciting milestone for experimental strong-field physics.”

Professor Stuart Mangles, who led the project, said: "These results give us a crucial benchmark for refining the quantum models that describe strong-field QED effects. These models underpin our understanding of both intense laser-plasma interactions and some of the most extreme astrophysical environments.

"By improving them, we can not only deepen our knowledge of these fundamental processes but also guide the development and applications of next-generation high-power lasers. While our data clearly require quantum models to explain what we see, we don’t yet have the precision to distinguish between competing quantum theories, so there is still work to be done"

We have seen that in these extreme fields, a quantum theory is needed to describe how light and particles behave. This result has been many years in the making and is a really exciting milestone for experimental strong-field physics Eva Los Department of Physics

Confirming these effects validates strong-field quantum electrodynamics (SFQED), and helps scientists to better understand how light and matter behave under these electric and magnetic fields.

The implications go far beyond the lab, and their results will help shape the design of next-generation laser facilities and advanced particle accelerators.

The team now have plans to compare different quantum models of radiation reaction to see which agree most closely with experiments. They will also study processes such as the creation of matter-antimatter particles from vacuum in very strong fields.

Even more extreme environments will become accessible as new ultra-high power lasers, such as Vulcan 20-20 at the Central Laser Facility (CLF) at Rutherford Appleton Laboratory, come online.

Professor Mangles added, "As we build even more powerful lasers, such as Vulcan 20-20 in the UK (which will be around forty times more powerful than Gemini), we will push into a regime where even current theories may start to break down. That’s both a challenge and a really exciting opportunity for the field."