2 March 2015

Image captures light as both a particle and a wave

by Will Parker

Quantum mechanics tells us that light can behave simultaneously as a particle or a wave, but it's only possible to see one of these attributes at any given moment. Even though a variety of experiments have successfully observed both behaviors of light, they have never been able to observe both at the same time. Now, Europeans scientists have been able to take the first ever snapshot of light behaving both as a wave and as a particle, demonstrating that imaging quantum phenomena at the nanometer scale is possible. The work, by EPFL scientists, has been published in Nature Communications.

The research team, led by Fabrizio Carbone, achieved the breakthrough by employing a clever twist: they used electrons to image light.

First, the researchers fired a pulse of laser light at a tiny metallic nanowire which added energy to the charged particles in the nanowire, causing them to vibrate. Light travels along this tiny wire in two possible directions and when waves traveling in opposite directions meet each other they form a new wave that looks like it is standing in place. This standing wave, radiating around the nanowire, becomes the source of light for the experiment.

This is where the experiment's trick comes in: 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 an ultrafast microscope to image the position where this change in speed occurred, Carbone's team could now visualize 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 demonstrated its particle aspect as well. As the electrons pass close to the standing wave of light, they "hit" the light's particles, 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 occurrence of these energy packets 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 Carbone. "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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