Fabrizio Carbone/EPFL

The weird way that light can behave as both a wave and a particle has been known to physicists for about a century, but a team in Lausanne has only now managed to capture it doing both at the same time.

The achievement comes after decades of attempts which successfully observed both wave-like and particle-like behaviour.


But the physics researchers at the École polytechnique fédérale de Lausanne took a radically different approach, using one of the only two ultrafast energy-filtered transmission electron microscopes that exist in the world.

They first fired a laser at a metallic nanowire, making charged particles inside vibrate. The light waves travel along the nanowire in two directions, but when they meet they form a 'standing' wave that remains stationary -- creating the source of light for the experiment.

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Then comes the novel part -- the physicists shot a stream of electrons close to the nanowire. As those electrons interacted with the light source, hitting the confined photons, they either sped up or slowed down, and the microscope allowed the researchers to image the position where the change in speed -- nd therefore the standing wave -- occurred. That shows the wave nature of light.

But at the same time, the impact of the electrons on the photons and the resulting change in speed (showing up as an exchange of energy in discrete 'packets', or quanta) shows that the light on the nanowire behaves as a particle. "This experiment demonstrates that, for the first time ever, we can film quantum mechanics -- and its paradoxical nature -- directly," says Fabrizio Carbone, who led the team.


Fabrizio Carbone/EPFL - Annotations by Wired.co.uk

We can explain what you're seeing in the top image in the annotated version here. It shows a graph where one axis (let's call it x) shows distance along the wire, the another (let's call it y) is the change in energy from the collisions, and the height (and colour) shows how often the collisions happen.

The change in colour along the x axis shows the standing waves that display the wave nature of light, while the change in colour along the y axis shows discrete packets of energy that display light's particle nature -- the first row shows the electrons that have gained the energy of one photon, the second two, and so on. In short, if it wasn't a wave, the colour wouldn't vary on the x axis, and if it wasn't a particle the colour wouldn't vary on the y axis.


The practical applications for this research into fundamental physics are somewhat distant, but could help unlock the power of quantum computers. "Being able to image and control quantum phenomena at the nanometer scale like this opens up a new route towards quantum computing," said Carbone.

The research, which was a collaborative effort between EPFL, Trinity College and the Lawrence Livermore National Laboratory, has

been published in the journal Nature Communications.