Over 200 years ago, English scientist Thomas Young performed a famous test known as thedouble slot experience”. He shone a beam of light onto a screen with two slits and observed that the light passing through the openings formed a pattern of dark and bright bands.
At the time, the experiment was supposed to demonstrate that light was a wave. The “interference pattern” is caused by light waves passing through the two slits and interfering with each other on the other side, producing bright bands where the peaks of the two waves align and dark bands where a peak encounters a trough and the two cancel each other out. .
In the 20th century, physicists realized that the experiment could be adapted to demonstrate that light behaves not just like a wave, but also like a particle (called a photon). In quantum mechanical theory, this particle always has wave properties – so the wave associated with a single photon passes through both slits and creates interference.
In a new twist on the classic experience, we’ve replaced screen slits with “slits” in time – and discovered a new kind of interference pattern. Our results are published today in natural physics.
Slots in time
Our team, led by Riccardo Sapienza of Imperial College London, has shone light through a material that changes its properties in femtoseconds (quadrillionths of a second), only letting light through at specific times in rapid succession. .
We have always seen interference patterns – but instead of manifesting as bright and dark bands, they have manifested as changes in the frequency or color of the beams of light.
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To carry out our experiment, we devised a way to activate and deactivate the reflectivity of a screen incredibly quickly. We had a transparent screen that became a mirror for two brief moments, creating the equivalent of two time slits.
So what are these time slits doing to the light? If we consider light as a particle, a photon sent to this screen could be reflected by the first increase in reflectivity or by the second, and reach a detector.
However, the wave nature of the process means that the photon is somehow reflected by both time slots. This creates interference and a variable color pattern in the light that reaches the detector.
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The amount of color change is related to the rate at which the mirror changes its reflectivity. These changes must be on time scales comparable to the length of a single cycle of a light wave, which is measured in femtoseconds.
Electronic devices can’t run fast enough for that. So we had to use the light to turn on and off the reflectivity of our screen.
We took a screen made of indium tin oxide, a transparent material used in cell phone screens, and made it reflective with a brief pulse of laser light.
From space to time
Our experiment is a great demonstration of the physics of waves and also shows how we can transfer concepts such as interference from the space domain to the time domain.
The experiment also helped us to understand materials capable of finely controlling the behavior of light in space and time. This will have applications in signal processing and perhaps even in light-powered computers.