X-ray Four-Wave Mixing Reveals How Electron Move Together Inside Atoms and Molecules
Directly Observing Electron Coherence for the First Time
Scientists working at the SwissFEL X-ray free-electron laser have achieved a long-standing experimental ambition in physics: directly revealing how electrons move in step with one another. Using a technique known as X-ray four-wave mixing, the team has opened a new window into the way energy and information travel through atoms and molecules.
Published in Nature, the research could one day shed light on how quantum information is stored and lost, helping to guide the development of more robust and error-resistant quantum technologies.
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Why Electron Interactions Matter
Much of the behaviour of matter arises not from individual electrons acting alone, but from their complex mutual interactions. Across chemistry and advanced materials, these interactions determine how molecules rearrange, how substances conduct or block electricity and how energy is transported.
In many emerging quantum technologies, including quantum computing, information is encoded in fragile patterns of interaction known as coherences. When these coherences decay, information is lost through a process called decoherence—one of the greatest obstacles facing practical quantum devices today.
Until now, although scientists have had many tools to probe the behaviour of individual electrons, the subtle coherences linking them have largely remained hidden.
A New Way to See Electron Coherence
Researchers at SwissFEL, based at the Paul Scherrer Institute (PSI) and EPFL in Lausanne, working with colleagues from the Max Planck Institute of Nuclear Physics in Germany and the University of Bern, have now found a way to reveal these connections using X-ray four-wave mixing.
"We can see how electrons dance together—whether they move in pairs or perform solo," explains Gregor Knopp, senior scientist at PSI's Centre for Photon Sciences and leader of the study. "This offers a fresh perspective on quantum phenomena and has the potential to transform our understanding of matter."
Like NMR, but with X-rays
In concept, X-ray four-wave mixing closely resembles nuclear magnetic resonance (NMR), a technique widely used in hospitals today for MRI scans. Both methods rely on sequences of pulses to generate and detect coherences within matter.
Four-wave mixing itself is already a well-established tool at infrared and visible wavelengths, where it enables scientists to explore how molecules move, vibrate and interact—supporting applications that range from optical communications to biological imaging.
X-rays extend this powerful approach down to far smaller scales, opening a direct window into the world of electrons.
"While many techniques reveal how atoms or entire molecules interact with each other or their environment, X-rays regime allow us to zoom in precisely on the electrons themselves," explains Ana Sofia Morillo Candas, first author of the study.
This ability could reshape research not only in quantum information science, but also in biology, solar-cell development and advanced battery materials.
Science, environment and advanced materials research
The 'Impossible' X-ray Experiment
Turning this type of X-ray experiment into a working reality had, until now, remained almost impossible—despite being proposed decades ago.
In four-wave mixing, three incoming light waves interact with matter to generate a fourth. "To perform four-wave mixing, different light beams must be spilt, precisely delayed and then recombined," explains Morillo Candas.
"With X-rays, this becomes exceptionally difficult because the wavelengths are so short—you need extraordinary precision."
In simple terms, manipulating three X-ray beams is like throwing three darts from a kilometer away and having them land within nanometers of each other.
Even this extreme precision is not enough. The signal produced is extraordinarily faint, requiring exceptionally bright and ultrashort flashes of X-ray light —available only at facilities like SwissFEL.
A Simple Idea Unlocks a Breakthrough
The breakthrough hinged on an unexpectedly simple idea borrowed from optical laser physics: an aluminum plate pierced with four microscopic holes.
- Three X-ray beams pass through separate apertures
- When perfectly aligned, a new X-ray signal emerges from the fourth
"It's conceptually very straightforward," says Knopp. "If you were working with infrared or visible light, this is exactly how you would do it."
Despite differing sharply from previous attempts, the method proved remarkably effective. "We were astonished by how strong the signal turned out to be," Knopp adds.
A Light in the Night' Moment
The decisive moment came in the middle of the night, when Morillo Candas—then a postdocotoral researcher at PSI —noticed the signal at the Maloja experimental station at SwissFEL.
"It glowed like a tiny light on the screen," she recalls. "To anyone else, it would have looked like nothing. But we leapt with joy."
From First Signal to Future Technology
The first successful demonstration of X-ray four-wave mixing was carried out in neon, a noble gas chosen for its simplicity and well-understood electron behaviour.
With this proof of principle established, researchers are now preparing to study:
- More complex gases
- Liquids
- Solid materials with rich electron interactions
Morillo Candas and Knopp believe the simplicity of the method will accelerate its adoption across other X-ray laser facilities.
Why This Matters for Quantum Technology
In the longer term, the technique could evolve into a powerful imaging tool, capable of showing where quantum coherences persist—and where they fail—inside materials and devices.
Such insights could:
- Reveal where quantum information is stored
- Identify where and how information is lost
- Guide the design of more stable qubits
- Reduce errors in future quantum computers
Quantum science, health & future technologies
A New Chapter in Quantum Imaging
"If, in the 1960s, someone had asked whether you could do an NMR scan of a knee, the response would have been confusion," Knopp says. "But it all began with a first signal. That's where we are today."
Looking ahead, X-ray four-wave mixing could one day become a routine imaging tool for tiny quantum devices, transforming how scientists observe and control the quantum world.
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