Scientists at the X-ray free-electron laser SwissFEL have realised a long-pursued experimental goal in physics: to show how electrons dance together. The technique, known as X-ray four-wave mixing, opens a new way to see how energy and information flow within atoms and molecules. In the future, it could illuminate how quantum information is stored and lost, eventually aiding the design of more error-tolerant quantum devices. The findings are reported in Nature.
Much of the behaviour of matter arises not from electrons acting alone, but from the ways they influence each other. From chemical systems to advanced materials, their interactions shape how molecules rearrange, how materials conduct or insulate and how energy flows.
In many quantum technologies – not least quantum computing – information is stored in delicate patterns of these interactions, known as coherences. When these coherences are lost, information disappears – a process known as decoherence. Learning how to understand and ultimately control such fleeting states is one of the major challenges facing quantum technologies today.
Until now, although many techniques let us study how individual electrons behave, we have mostly been blind to these coherences. Scientists at SwissFEL from the Paul Scherrer Institute PSI and Swiss Federal Institute of Technology in Lausanne (EPFL), in collaboration with the Max Planck Institute of Nuclear Physics in Germany and University of Bern, have now developed a way to access them using a technique known as X-ray four-wave mixing.
“We learn how the electrons dance with each other – whether they hold hands, or if they dance alone,” says Gregor Knopp, senior scientist in the Center for Photon Sciences at Paul Scherrer Institute PSI, who led the study. “This gives us a new view on quantum phenomena and can change how we understand matter.”
Like NMR, but with X-rays
Conceptually, X-ray four-wave mixing is similar to nuclear magnetic resonance (NMR), which today is used daily in hospitals for MRI scans. Both techniques use multiple pulses to create and read out coherences in matter.
The process of four-wave mixing is also already well-established using infrared and visible light, where it allows scientists to investigate how molecules move, vibrate and interact with one another – with applications ranging from optical communications to imaging biological samples.
X-rays bring this same kind of powerful approach to a smaller scale and allow us to step into the world of the electrons. “Whereas other approaches tell us about how atoms or molecules as a whole interact with each other or with their surroundings, with X-rays we can zoom right in to the electrons,” says Ana Sofia Morillo Candas, first author of the paper.
This ability to zoom in on the interactions between electron has the potential to provide completely new insights not only into quantum information, but also into many other areas – for example biological molecules or materials for solar cells and batteries.
ow you would do it.” This approach is very different to previous attempts made at X-rays four-wave mixing, but to Knopp, it seemed like the obvious method to try. “We were amazed when we saw how large the signal was,” he adds.
It was the middle of the night, when Morillo Candas, at that time a postdoc at PSI, saw the signal in the control room of the Maloja experimental station at SwissFEL. She remembers: “It glowed like a light on the screen. To anyone else, it would look like nothing. But we jumped for joy.”
Read more on the PSI website
Image: Artistic impression of X-ray four-wave mixing – a technique that reveals how electrons interact with each other or with their surroundings. The ability to access this information is important for many fields: from understanding how quantum information is stored and lost to designing better materials for solar cells and batteries.
Credit: © Noah Wach
