Scientists demonstrate quantum recoil for the first time, paving the way for precise X-ray imaging

Nanyang Asst Prof Wong Liang Jie (left) and research fellow Dr Huang Sunchao from Nanyang Technological University, Singapore with a scanning electron microscope used in their quantum recoil experiments. (Credit: NTU Singapore)

For the first time since it was proposed over 80 years ago, scientists from Nanyang Technological University, Singapore (NTU Singapore) have demonstrated the phenomenon of “quantum recoil”, which describes how the particle nature of light has a major impact on electrons moving through materials. The research is published online today in the journal Nature Photonics.

Making quantum recoil a practical reality should eventually allow businesses to more accurately produce X-rays of specific energy levels, leading to superior accuracy in healthcare and manufacturing applications such as medical imaging and flaw detection in semiconductor chips.

A decades-old theory

Quantum recoil was theorised by Russian physicist and Nobel laureate Vitaly Ginzburg in 1940 to accurately account for radiation emitted when charged particles like electrons move through a medium, such as water, or materials with repeated patterns on the surface, including those on butterfly wings and graphite.

This radiation is created when the moving electrons disturb atoms in the medium or the material. As the atoms return to an undisturbed state, they emit radiation, such as X-rays.

Although the electrons are supposed to lose energy and slow down when this happens, classical theory predicts that its impact on the emitted radiation is negligible.

However, Ginzburg posited that this assumption breaks down when considering the quantum electrodynamics theory that deals with how charged particles interact with an electromagnetic field, as well as how light interacts with matter.

According to the theory, when moving electrons decelerate after disturbing nearby atoms, the energy and momentum these electrons lose should be transferred to the radiation emitted. This happens because light exists as particles that have energy and momentum, and which move in a wave as radiation.

The transfer causes the energies of the radiation released to shift from classical predictions and also affects the slowing electrons by causing them to deviate from their path of travel.

This phenomenon is known as quantum recoil. However, no one has been able to experimentally prove it – until now.

The Singapore scientists, led by Nanyang Assistant Professor Wong Liang Jie from NTU’s School of Electrical and Electronic Engineering, demonstrated the phenomenon through separate experiments that bombarded electrons from a scanning electron microscope onto two very thin materials, about 1,000 times thinner than a strand of hair.

The two materials were boron nitride in a hexagonal form often used as a lubricant in paints, and graphite which is used to make rechargeable battery terminals and also found in pencil lead.

Using an energy dispersive X-ray spectrometer detector, the researchers measured the X-rays emitted from this bombardment and found that they had energies that were different from those predicted by classical theory but could instead be explained by quantum recoil.

In one set of experiments, X-rays with an energy of 1,300 electron volts (eV) were measured, as predicted by quantum recoil theory. Classical theory postulated that X-rays with an energy of 1,340 eV would be detected. The 40 eV difference showed that the X-rays lost energy in reality.

The researchers said that the quantum recoil effect could be much larger under certain conditions, such as when the materials bombarded by the electrons are tilted or if the direction from which the X-rays are observed is changed.

Their calculations suggest that if an emitted X-ray with a classically predicted energy of around 300 eV is observed in a few specific directions, the measured energy could drop to almost zero due to quantum recoil, although further research is needed to confirm this.

Paradigm shift

Scientists have been trying to conduct experiments for years to prove quantum recoil but have hit various snags.

One reason is that, in theory, to produce the effect at a level that is measurable, the materials that the electrons hit need to have tiny groove-like surface patterns repeated at very high intervals. It means that the distance between patterns needs to be less than 1 nanometre (nm), which is hundreds of thousands of times smaller than the diameter of a strand of hair.

Researchers have been trying to create these small patterns artificially with nanofabrication methods, such as those using laser beams. However, these methods have so far been able to create patterns with a distance of around 10 nm between them, which Asst Prof Wong said is already a huge achievement.

But he and his team made a paradigm shift that helped them bypass the current fabrication limitations – they used existing materials that naturally have such minute patterns.

The NTU scientists’ photonics research experience in X-rays played a big role in this, said Asst Prof Wong. Because they deal with X-rays that have wavelengths ranging between 0.01 nm and 10 nm, he and his colleagues constantly think about performing experiments at such scales, which coincides with the requirements for proving quantum recoil.

“Thinking of variations in materials on the atomic scale is natural since we work with X-rays that have wavelengths that are also on the atomic scale,” he said.

This led them to experiment with boron nitride and graphite as the researchers knew that these two materials are made of atoms in very regular arrangements. In particular, they consist of many ultra-thin layers each just one atom thick that form naturally occurring groove-like patterns repeating at intervals of less than 1 nm, the amount required to demonstrate quantum recoil in a measurable way. Their experimental results proved that they were right.

Professor Peter Baum from Germany’s University of Konstanz, who was not involved in the study but reviewed the research paper, said that the findings of the Singapore scientists are fundamentally significant. 

"This work illuminates a so far unknown regime where free electrons interact with free photons with the help of a crystal," said Prof Baum, whose research focuses on the interaction of light and matter at the atomic scale.

He was referring to crystalline materials whose atoms are arranged in highly ordered and repeated patterns. They include the hexagonal boron nitride and graphite tested in the NTU experiments.

Practical implications