Light observed as both a particle and a wave

Since the days of Albert Einstein, scientists have recognised that light can behave simultaneously as a particle or a wave. But there has never been an experiment able to capture both natures of light at the same time - until now.

When UV light hits a metal surface, it causes an emission of electrons. Einstein proposed that this ‘photoelectric’ effect was a result of light being a stream of particles as well as a wave. A research team led by Fabrizio Carbone at EPFL has now utilised electrons to take a snapshot of light behaving as both a wave and a particle. Their results have been published in the journal Nature Communications.

The experiment began with a pulse of laser light being fired at a tiny metallic nanowire. The laser adds energy to the charged particles in the nanowire, causing them to vibrate. Light travels along this wire in two possible directions, like cars on a highway. When waves travelling in opposite directions meet each other, they form a new wave that looks like it is standing in place. This standing wave becomes the source of light for the experiment, radiating around the nanowire.

The scientists then shot a stream of electrons close to the nanowire. As they passed close to the standing wave of light, they ‘hit’ the light’s particles (photons), making them move faster or slower. This change in speed appears as an exchange of energy packets (quanta) between electrons and photons, whose occurrence shows that the light on the nanowire behaves as a particle. The team used an ultrafast transmission electron microscope to image the position where this change in speed occurred, visualising the standing wave - and thus the wave-nature of light - in the process.

© 2015 EPFL.

“This experiment demonstrates that, for the first time ever, we can film quantum mechanics - and its paradoxical nature - directly,” said Carbone. He added that the work could have implications for future technologies, stating, “Being able to image and control quantum phenomena at the nanometre scale like this opens up a new route towards quantum computing.”

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