Shining Light Through Double-Slits In Time

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Issue 15, Volume 113

By Sophie Zhao 

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Imagine sitting by a lake with pebbles scattered nearby. You proceed to toss a pebble into the water, creating ripples. This is a common example of a wave, but a wave is more than just a cool phenomenon that results from throwing pebbles into water. A wave is defined as a disturbance that propagates through a medium. Thus, the ripples that appear after the pebble penetrates the water’s surface are a series of disturbances propagating outward. However, water waves are not the only type of waves.

The wave-like behavior of light was demonstrated by Thomas Young in his famous double-slit experiment in 1801. In waves, the highest points are called crests, or peaks, while the lowest points are called troughs. When waves meet, destructive and constructive interference occur. Peaks and troughs of different waves annihilate each other in destructive interference, while corresponding points (two peaks or two troughs) enhance each other in constructive interference. Repeating incidents of destructive and constructive interference create an interference pattern. Young’s experiment demonstrated that light behaves like waves by revealing its interference pattern.

Young set up an opaque wall with two narrow slits for light to pass through. He shined a beam of light at the wall and used a detector to record the photons that were able to pass through the slits. When both slits of the wall were opened, an interference pattern was created on the screen behind the wall. The screen had a repeating series of areas that were heavily bombarded by photons where constructive interference occurred, as well as areas with no recorded photon hits. These empty areas were most likely where the peaks and troughs of light engaged in destructive interference. Hence, Young concluded that photons exhibit wave-like behavior, interfere with each other, and produce their own interference patterns.

Young’s groundbreaking double-slit experiment explored splits in space—the wall he used had physical openings that restricted the flow of light. However, in April 2023, a team of physicists from Imperial College London added a creative twist to this experiment by using “slits in time.” In other words, the team restricted the flow of light to certain periods of time. Their experiment references Albert Einstein’s theory of special relativity, stating that space and time are inevitably linked. If Young’s experiment worked with spatial slits, the researchers thought: Wouldn’t it also work for temporal slits?

The Imperial College London team recreated the double-slit experiment using a thin film of indium tin oxide as the wall. Indium tin oxide is an example of a metamaterial, a material that does not occur naturally. The scientists fired a laser at the wall at consistent time intervals every few femtoseconds (10-15 of a second) so the beam of light could only pass through and hit the screen in short bursts. Since the ability of light to pass through this wall is limited by the timing, the slits are referred to as time slits.

When light passes through the wall in between periods of the wall’s reflectivity, the light’s frequency is altered, leading to a change in color. Lights with the same frequencies—and hence the same colors—enhance each other, while lights with different frequencies and colors cancel each other out. The series of annihilations and enhancements forms an interference pattern. Though both Imperial College London’s and Young’s experiments produced an interference pattern, the way it formed differed in each experiment. When light came out of Young’s spatial slits, what changed was the angle at which the light traveled rather than the frequency of light. This resulted in waves colliding with each other. Nonetheless, Imperial College London’s experiment still demonstrated the wave-like behavior of light.

This study’s twist on the double-slit experiment has numerous important implications. First, Imperial College London’s experiment corroborates the linkage between space and time, as it provided consistent evidence of light’s wave behavior in both domains. This consistency supports Einstein’s theory and ushers us to view time and space not as separate entities, but as interconnected phenomena. Additionally, the use of indium tin oxide opens up exploration for other practical uses of metamaterials. For example, similar materials that change light permeability under certain conditions can serve as optical switches that enhance the efficiency of operations by computers. Metamaterials may enhance computational efficiency to a level that even compares with human brain functioning. Overall, Imperial College London’s experiment offered the scientific community a new perception of light and space-time, opening new doors for quantum technology and theoretical physics.