The concept of photons -- that is, particles of light -- has been around since 1905, explained by Albert Einstein as the "photoelectric" effect. This effect occurs when light hits a metal surface, causing that metal surface to emit photoelectrons. However, the behaviour of these electrons could not be accounted for if the light was a wave -- it only made sense if the light was made up of particles.
It has been since demonstrated that light behaves as both. It has been observed behaving as a wave, and it has been observed behaving as a particle -- although it had never, until now, been directly observed doing both at the same time.
The achievement has been made by researchers at the École polytechnique fédérale de Lausanne, led by Fabrizio Carbone -- employing a novel technique that uses electrons to image light.
"This experiment demonstrates that, for the first time ever, we can film quantum mechanics -- and its paradoxical nature -- directly," Carbone said.
To take the photo, the team used special apparatus, firing a pulse of laser light at a tiny metallic nanowire, which contained charged particles. The laser energised those particles, inducing them to vibrate. Light waves were then sent along the wire in two opposite directions, meeting in the middle and creating a third wave -- a standing wave.
This is when the stream of electrons was introduced, fired close to the nanowire and its confined standing light wave. As they reached the light, the electrons either sped up or slowed down; and, using an ultrafast microscope, the team could image the standing wave.
But the electrons also showed the standing wave contained particles. As the electrons came into contact with the wave, they collided with the photons which resulted in a change of speed, caused by an exchange of energy between the photons and the electrons.
It is this change of speed -- and the energy exchange it proves -- that in turn demonstrates the particle behaviour within the standing wave.
This, Carbone said, has implications and possible future applications in computing. "Being able to image and control quantum phenomena at the nanometer scale like this opens up a new route towards quantum computing," he said.
The full paper, "Simultaneous observation of the quantization and the interference pattern of a plasmonic near-field," can be found online in the journal Nature Communications.