Light interacts with its past self in twist on double-slit experiment

The famous double-slit experiment, which demonstrated that light is both a wave and a particle, has been performed using “slits in time”. The techniques involved present a new way to manipulate light that could be used to create strange materials called time crystals.

The double-slit experiment, first performed by Thomas Young in 1801, involves shining a beam of light on a plate or card with two small slits cut into it for the light to pass through. When the light waves pass through the slits, they interfere with one another, causing a pattern of light and dark stripes on a screen. This wouldn’t be possible if light were simply made of particles, so this experiment was one of the first pieces of evidence that light is a wave as well.

While the original double-slit experiment used two slits separated in space, Riccardo Sapienza at Imperial College London and his colleagues performed a similar experiment where the obstacles to the light’s propagation were separated in time. “The temporal manipulation of waves is an old subject, but it’s been mostly driven by theory for the last 30 years,” says Sapienza. “It has been very hard to do experiments, especially with light.”

That’s because doing such experiments requires materials that can change from being transparent to reflective with extraordinary speed to create what the researchers call “slits in time”. Sapienza and his team used a material called indium tin oxide, which is commonly used in coatings for various electronic displays. When it is hit with a powerful laser beam, it goes from being almost entirely transparent to briefly reflecting most of the light that hits it.

To perform the experiment, the researchers used two consecutive laser pulses to turn the material reflective while also shining a less powerful “probe” laser at it. The light from the probe laser passed through the material during times when it was not reflective, and bounced back when it hit simultaneously with a laser pulse.

When they measured the light that bounced back, the researchers found similar interference patterns to those seen in the classic version of the experiment, but this time in the frequency of the light, which determines its colour, rather than in its brightness. “In the Young experiment, light enters at one angle and comes out at many angles, and in our experiment the light enters at one frequency and comes out at many frequencies,” says Sapienza.

This was as theoretical calculations predicted, but the light’s frequency oscillated much more than the researchers expected. The number of oscillations depends on the sharpness of the material’s transition from transparent to reflective, so this means that the material was responding to the laser pulses with incredible speed – within a few femtoseconds of the pulse. One femtosecond is one-millionth of one-billionth of a second.

“The material response is 10 to 100 times faster than expected, and that was a big surprise,” says Sapienza. “We were hoping to see a few oscillations, and we saw many.”

That quick transition time could be useful for making time crystals, which are strange materials with moving structures that repeat over and over again. It could also help with more everyday applications, says Maxim Shcherbakov at the University of California, Irvine. “The temporal interference is an exciting find that can see applications in many modern technologies but especially in telecommunications, where the way we treat signals in time is very important,” he says.

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