Scientists image light as both particle and wave simultaneously

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Scientists from the Swiss Federal Institute of Technology in Lausanne (EPFL) have taken the first ever photograph of light behaving as both a particle and wave. The EPFL team, whose work is published in Nature Communications, used electrons as a means to image both aspects of light, which, according to the researchers, could even facilitate the development of quantum computing.

Quantum mechanics tells us that light can behave simultaneously as a particle or a wave.

When UV light hits a metal surface, it causes an emission of electrons. Albert Einstein explained this ‘photoelectric’ effect by proposing that light – thought to only be a wave – is also a stream of particles.

However, although scientists have been able to observe both the particle and wave behaviour of light, there has never been an experiment able to capture both natures of light at the same time.

A research team led by Fabrizio Carbone used EPFL’s ultrafast energy-filtered transmission electron microscope – one of only two in the world − to image both behaviours simultaneously.

‘This experiment demonstrates that, for the first time ever, we can film quantum mechanics – and its paradoxical nature – directly,’ said Fabrizio Carbone.

During the experiment, the EPFL team fired a pulse of laser light at a tiny metallic nanowire, causing the charged particles in the nanowire to vibrate. The light travels along the nanowire in two possible directions, and when waves travelling in opposite directions meet each other they form a new wave that looks like it is standing in place. Here, this standing wave becomes the source of light for the experiment, radiating around the nanowire.

The scientists shot a stream of electrons close to the nanowire, using them to image the standing wave of light. As the electrons interacted with the confined light on the nanowire, they either sped up or slowed down. Using the ultrafast microscope to image the position where this change in speed occurred, Carbone’s team could now visualise the standing wave, which acts as a fingerprint of the wave-nature of light.

While this phenomenon shows the wave-like nature of light, it simultaneously demonstrates its particle aspect as well. As the electrons pass close to the standing wave of light, they ‘hit’ the photons. This affects their speed, making them move faster or slower.

This change in speed appears as an exchange of energy ‘packets’ − quanta − between electrons and photons. The very occurrence of these energy packets shows that the light on the nanowire behaves as a particle.

Imaging quantum mechanics and its paradoxical nature could extend beyond fundamental science and to future technologies, Carbone explained: ‘Being able to image and control quantum phenomena at the nanometer scale like this opens up a new route towards quantum computing.’

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Related links

Article in Nature Communications

EPFL's Laboratory for ultrafast microscopy and electron scattering