Famous double slit experiment recreated in the fourth dimension by physicists

More than 200 years ago, English scientist Thomas Young performed a famous test known as the “double slit experiment”. He shone a beam of light on a screen with two slits in it and saw that the light passing through the openings formed a pattern of dark and bright bands.

At the time, it was believed that the experiment showed that light was a wave. The “interference pattern” is caused by light waves passing through both slits and interfering with each other on the opposite side, creating bright bands where the crests of the two waves align and dark bands where a peak meets a trough and the two cancel out .

In the 20th century, physicists realized that the experiment could be modified to show that light behaves not only as a wave, but also as a particle (called a photon). In quantum mechanical theory, this particle still has wave properties — so the wave associated with even one photon passes through both slits, causing interference.

In a new twist on the classic experiment, we replaced the slits in the screen with “slits” over time – and discovered a new kind of interference pattern. Our results are published today in Natural Physics.

Cracks in time

Our team, led by Riccardo Sapienza of Imperial College London, fired light through a material whose properties change in femtoseconds (quadrillionths of a second), allowing light to pass only at certain times in rapid succession.

We still saw interference patterns – but instead of showing up as light and dark bands, they showed up as changes in the frequency or color of the light rays.

Read more: Curious Kids: Is light a wave or a particle?

To run our experiment, we came up with a way to turn a screen’s reflectivity on and off incredibly quickly. We had a transparent screen that became a mirror for two brief moments, creating the equivalent of two slits in time.

Color interference

So what do these slits in time do to light? Thinking of light as a particle, a photon transmitted on this screen can be reflected from the first increase in reflectivity or from the second and reach a detector.

However, the wave nature of the process means that the photon is in some sense reflected from both temporal slits. This causes interference and a varying color pattern in the light reaching the detector.

Read more: Explainer: what is wave-particle duality

The rate of change in color is related to how quickly the mirror changes its reflectivity. These changes should occur on time scales comparable to the length of a single light wave cycle, measured in femtoseconds.

Electronic devices cannot function fast enough for this. So we had to use light to switch the reflectivity of our screen on and off.

We took a screen made of indium tin oxide, a transparent material used in mobile phone screens, and made it reflective with a short pulse of laser light.

From space to time

Our experiment is a wonderful demonstration of wave physics and also shows how we can transfer concepts such as interference from the domain of space to the domain of time.

The experiment also helped us understand materials that can minutely control the behavior of light in space and time. This will have applications in signal processing and maybe even light-powered computers.