Quantum Computing: The Double Slit Experiment

Seven weeks ago, I started a course offered by IBM and The Coding School on Quantum Computing. At the time I really didn't know much about Quantum Mechanics(I still don't). I had read fascinating articles about Schrödinger's Cat and qubits(quantum bits) being a combination of both 0 and 1 at the same time, but I had no clue as to how these articles could be true and what the implications of these phenomena are.

A simple definition for Quantum Computing is the use of quantum mechanics to manipulate qubits in favorable ways. An important property of quantum mechanics is called 'interference' and a perfect example of it is found in the Double Slit and Quantum Eraser experiments.

The Double Slit Experiment:

If you're like most people, you probably think of light as something that allows us to see. You've might have heard that different colors of light exist as waves with different lengths. However, you might have also heard that light exists as individual particles called photons that bounce off objects, like mirrors, in predictable patterns. So what is light? A wave? A particle?    

Imagine cutting two slits in a box and shinning a light perpendicular to the box. You would probably expect to see two corresponding groups of light that match the shape of the slits. However, you might instead see something like this:

(A double-slit interference of the sunlight passing through two slits ~1 cm long and ~0.5 mm apart. At the top and the bottom of the image the interference on the edge of the slit produces noticeable variation of the brightness. Taken by Aleksandr Berdnikov, https://en.wikipedia.org/wiki/Double-slit_experiment#/media/File:Double_slit_interference.png)

Believe it or not, this image is simply the result of sunlight passing through two slits. It's a good example of the result of a Double Slit experiment. In the rest of this post I'll do my best to explain why this is happening. If you'd like to see an interesting video that does a great job of conducting and explaining this experiment, here's a personal favorite by Veritasium on Youtube:

Thomas Young conducted a Double Slit Experiment as early as 1801, below is a basic diagram:

(Diagram of a Thomas Young Style Double Slit Experiment. Source: https://i2.wp.com/unfoldanswers.com/wp-content/uploads/2020/06/Double-Slit-Experiment.jpg?w=940&ssl=1)

The results of this experiment suggest that light acts as a wave. To understand why, it's important to know some basic properties of waves.

(Diagram of a wave. Source: https://physics.stackexchange.com/questions/291571/how-is-the-combination-of-electric-magnetic-waves-electromagnetic-wave-illus)

Waves are cyclical by nature and we call the distance between cycles of a wave a Wavelength. A high frequency wave will have a low Wavelength, meaning that there will be many cycles at any given period. A low frequency wave will have a high Wavelength, meaning that there will be few cycles at any given period. High points in a wave are called crests while low points in a wave are called troughs. The amplitude of a wave is the height of it's crests or troughs. 

Knowing this we can finally talk about interference. When waves collide they 'interfere' with each other. When a crest combines with a crest or a trough combines with a trough, they combine to create an even greater crest or trough; this is called constructive interference. When a crest combines with a trough they cancel out and collapse; this is called destructive interference.

When you look at images of interference patterns from a double-slit experiment you are seeing the result of waves interfering with each other. In the particular example of light, the biggest crest/trough combinations are the brightest and shown in the center of the image. As you look towards the edges, you notice that the size of these combined crests/troughs vary in brightness(and therefore amplitude). You may also notice different colors at the edges of the pattern. This is caused by the combinations of different light wavelengths. The bars of darkness you see in-between are caused by the destructive interference of crests and troughs.

So this means that light acts as a wave right? Problem solved. Well... not quite. This is where things get weird. As mentioned earlier, light can also be described as the collection of individual photons. When you measure which slits the individual photons go through, suddenly this interference pattern collapses and you are left with..., guess what, two bars of light. Even more strangely, this collapse of the interference pattern can appear to occur retroactively.

This odd retroactive collapse can be observed in a variation of the Double Slit Experiment called the Quantum Eraser Experiment. I'll explain the details of this experiment in another post, which will reveal interesting and odd properties of quantum mechanics, such as quantum entanglement.