In a first, scientists have shown they can send light through “slits” in time.
The new experiment is a variation of a 220-year-old demonstration, in which light shines through two slits in a screen to create a unique diffraction pattern in space, where the peaks and valleys of the light wave add up or cancel each other out. In the new experiment, the researchers created a similar pattern over time, essentially changing the color of an ultra-short laser pulse.
The findings pave the way for advances in analog computers that manipulate data printed on beams of light instead of digital bits – it could even cause these computers to “learn” from the data. They also deepen our understanding of the fundamental nature of light and its interactions with materials.
For the new study, described April 3 in the journal Nature Physics (opens in a new tab), the researchers used indium tin oxide (ITO), the material found in most phone screens. Scientists already knew that ITO could change from transparent to reflective in response to light, but the researchers found that this happened much faster than previously thought, in less than 10 femtoseconds (10 millionths of a billionth second).
“It was a very big surprise and at first it was something we couldn’t explain,” said the study’s lead author. Ricardo Sapienza (opens in a new tab), a physicist from Imperial College London, told Live Science. Eventually, the researchers figured out why the reaction happened so quickly by examining the theory of how electrons in the ITO react to incident light. “But it took us a long time to figure it out.”
Time trades for space
English scientist Thomas Young first demonstrated the wave nature of light using the now classic “double slit” experiment in 1801. When light shines on a two-slit screen, the waves change direction, so that the waves coming out of one slot overlap with the waves passing through the other. The peaks and troughs of these waves add up or cancel each other out, creating bright and dark fringes, called the interference pattern.
In the new study, Sapienza and his colleagues recreated such an interference pattern over time by shining a “pump” laser pulse onto an ITO-coated screen. While the ITO was initially transparent, the light from the laser changed the properties of the electrons in the material so that the ITO reflected light like a mirror. A subsequent “probe” laser beam hitting the ITO screen would then see this temporary change in optical properties as a time slit of only a few hundred femtoseconds. Using a second pump laser pulse caused the material to behave as if it had two time slits, an analog of light passing through double spatial slits.
While passing through conventional space slits causes light to change direction and scatter, when light passes through these twin “time slits”, its frequency changes, which is inversely proportional to its wavelength. It is the wavelength of visible light that determines its color.
In the new experiment, the interference pattern showed up as extra fringes or peaks in the frequency spectra, which are graphs of measured light intensity at different frequencies. Just as changing the distance between spatial slits changes the resulting interference pattern, the shift between temporal slits dictates the spacing of interference fringes in frequency spectra. And the number of fringes in these interference patterns that are visible before their amplitude decreases to the background level reveals how quickly the properties of ITO change; materials with slower responses produce fewer detectable interference fringes.
This isn’t the first time scientists have figured out how to manipulate light in time rather than space. For example, scientists from Google says its ‘Sycamore’ quantum computer created a time crystala new phase of matter that changes periodically over time, as opposed to atoms arranged in a periodic pattern in space.
Andrea Alu (opens in a new tab)a City University of New York physicist who was not involved in these experiments but did separate experiments that created reflections of light in time, described it as another “neat demonstration” of how time and space can be interchangeable.
“The most remarkable aspect of the experiment is that it demonstrates how we can change the permittivity [which defines how much a material transmits or reflects light] of this material (ITO) very quickly, and in large quantities,” Alù told Live Science via email. “This confirms that this material may be an ideal candidate for demonstrating time reflections and time crystals.”
Researchers hope to use these phenomena to create metamaterials or structures designed to alter the path of light in specific and often sophisticated ways.
Until now, these metamaterials have been static, which means that changing how the metamaterial affects the path of light requires the use of an entirely new metamaterial structure – a new analog computer for each different type of computation, e.g. example, said Sapienza.
“Now we have a material that we can reconfigure, which means we can use it for multiple purposes,” Sapienza said. He added that such technology could enable neuromorphic computing that mimics the brain.
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