The Double-Slit Experiment: Unveiling Quantum Mysteries

Without a doubt - AI and Quantum Computing were buzzed for last decades, but not really adopted on large scale. Things have changed recently with the exposure of ChatGPT and other generative AI tools "for free" to public (it's not really free, even if it looks like it's free, but that's another story). Boom for AI is enormous and you cannot disagree.

However - when it comes to quantum computing we are still 'living in a dream' and we await some event or breakthrough dicovery that will spin-off the explosion of the quantum adoption. Even though this might be my subjective wishfull thinking - I truly do believe that quantum computing is queued to be next worldwide revolution and moreover - that it's technically possible.

In order to wrap-up and understand just basic theory that stands behind quantum computing we first need to dvelve into some maths, physics and briliant minds of father and grandfathers of quantum mechanics. In this article I would like to come back in time to the early 19th century when Thomas Young performed probably the most important experiment for modern physics - the double-slit experiment - aimed for solving debate about the nature of light - is it a particle or a wave. Funny enough, Young was only partially right by proving that light is indeed a wave, but what's even crazier is that he was surely not aware about profund consequences of his discovery.

Original version of the experiment

Idea and setup is simple: shot a beam of light towards barier with two narrow vertical slits and examine image generated on back screen. There are two possible outcomes:

• If light is a particle, there should be two lines of light, aligned with the slits (figure 1)

• If light is a wave, there should be interference pattern (figure 2)

Before considering two slits it's interesting to start the experiment with one slit being covered. The result is a single line displayed on the screen - as expected. But this not proves anything. So let's open the second slit..

As I've already spoiled before - resulting image is an interference pattern as depicted in Figure 2.

Fact 1: Light is a wave

Just like water waves, initial light beem wave splits into two separate waves which interfere with each other, resulting with nice interfenece pattern.

Counterintuitvely if assumed that light is a particle: imagine pushing sand particles via two slits - you can try it yourself at beach - result would be definitely like in Figure 1, sand consits of small particles in the end? Sand is not a wave, or is it? Stay focused, cause things will get crazy in a second!

For Young that was it - he checked what he wanted to check, end of story. Light is a wave, so what? Well, the problem was that under some circumstances light was still indeed behaving like particles! Thankfully, 20th century physcists had a little bit better tools and were performing many variations of this experiment that ultimately turned out to provide much more bizzare observations.

One particular variation of this experiment resulted with so astonishing and unexpected outcome that it has turned upside down whole theoretical physics, leading to many confusions and open questions till today. Lucky for us - it also led to definition of quantum mechanics and eventually - quantum computing.

Let's detect where is the light

So what was the variation? Super simple! Physicists wanted to detect via which slit the light is passing by, using light detectors. As expected, light was detected in both slits. But when they've looked at the back screen they were speachless - interference pattern was gone, and light formed two vertical lines, aligned with the slits! Just like in Figure 1!

Fact 2: Light is a particle, if you observe it

This totaly unexpected result has proven that under some circumstances light indeed loses it's wave nature and start to behave as classical particles - similary to sand grains. The fact of observation instantly transformed light from wave to particle. Light is a wave, but it is also a particle. This phenomenon was named wave-particle duality.

What confused me once I was studying quantum physics and this particular version of the double-slit experiment was the definition of observation - what it means to observe/measure the light. Is there any way to trick the light beam by placing detectors in different places etc. - well, it turned out that any sort of detecion or measurement technique ends up with the loss of interference pattern. Academically saying - light wave collapses under any interaction with the external world. I will come back to wave funtion collapse in a second.

From wave-particle duality to the definition of quantum mechanics

Term "quantum" was introduced by Max Plank in 1900 (around 100 years after original double-slit experiment) when he was working on theory of black-body radiation (don't worry, I'm not gonna dive into black-body theory now). What's important is that he discovered that the only way to describe some observable effect was by introducing precisely defined chunk of energy, meaning that the energy must be carried in extremely small portions - which he named quants.

Soon after (1905) none other than famous Albert Einstein described photoelectric effect (fact the light beam can eject electrons from certain materials) by taking over Planck's idea of quants and introducing the name for the particles of light - photons.

Fact 3: Photon is a quant of light

Great, so Einstein has proven that light has indeed physically measurable particle nature, it consists of small particles called photons. Now let's go back to double-slit experiment and try another trick.

Shooting single photons - moment when things get really crazy!

Remember when we wanted to detect where is the beam of light and we've lost interference pattern due to the interaction with the light (the very fact of observation)? We can do another thing, as we can try to shoot single photons towards the slits (yes, we can do that) and detect via which slit the photon will go through. Can you guess the result?

Nothing crazy happens. If we measure how many photons go through each of the slit we have roughly 50/50 split, meaning that each single photon has 50% chance of going via first or second slit, and the resulting image consists of two verticle stripes, just like in Figure 1.

Light behaves as a particle - we are shooting single photons (quants), and we observe their position using detectors (via each slit it went through) - just like we would be shooting baseball balls via slightly bigger slits, one after one.

But wait - what if we don't observe the photons and still shoot sinle photons? Just like baseball balls or grains of sand we expect each photon to have 50% chance of going via one slit or another, so that the resulting image wlil be two stripes (Figure 1), right? Then watch this:

Figure 4 depicts timelaps of double-slit experiment when shooting single photon at time. Results are distrubing, surprising, unreal! We have interference pattern! But how? Why stream of single photons - definitely particles - behaves as wave? If single photon at one time is passing via single slit - with what it interferes? With itself!?

That's exactly the moment when quantum world starts to emerge in it's full form.

Fact 4: Single photon behaves as wave, if not observed

This is a lot to wrap up, I know. Single photon seems to split, go via both slits simultaneusly and interfere with itself. Moreover, if we try to look via each slit is goes - it 'decides' to not split and choose only one slit, thus no interference. Those are confirmed results of the double-slit experiment - unbelievable crazy!

Is it light only? What about other types of particles?

Is light and photon so special? Maybe we just need to accept that light is some magical unique form of matter in our universe that behaves so strangely?

No... We can try double-slit experiment, one last time (at least today) and shoot beam of single electrons - instead of photons - and check the results. Yeap, you've guessed it - we will get interference pattern, yet again!

Actually, we can even shoot single atoms or even whole molecules and we will alwyas have the same result - interference pattern! Meaning that all the matter is actually a wave - although, it's much more difficult to maintain such experiment with larger particles, as it's hard to 'not observe' the particles so the wave-nature collapses (which is by the way the exact reason why quantum computer is so difficult be built).

Light is no special at all.

Fact 5: We live in a quantum world

Do you remember your physics or chemistry classes in high school when we were taught Bohr's model of atom with central nucleus containing protons and neutrons surrended by electron shells / energy levels? That's simply a clever way to overcome quantum nature of the electron for young students so that their minds don't explode with the results of the double-slit experiment and the nature of quantum world.

Leverage quantum nature - spark for quantum computing

I want to conclude about implications of all of those discoveries for quantum computing. The fact that matter can exists in this special wave-like state, where it's position is unknown - proven by double-slit experiment that matter behaves as it is existing in all possible states simultaneusly (just with different probabilities, reflected by interference pattern) - can be used for computation. And this special state has even a name - superposition.

Superposition, together with entanglement and possibility of interference (changing probability distribution within superposition) are one of the core elements used by quantum computers to perform calculations. I don't wanna slaughter quantum computing now with oversimplification, but one of the fundamental and must-have funcionalities of quantum computers is capabilty to put qubits (quantum bits) into superposition (by applying Hadamard quantum gate).

More about superposition, entanglement, interference, qubits, quantum logic gates and many, many more will be covered in future posts in this newsletter.

Final thoughts

Double-slit experiment - with all of it's weirdness - proves, that our universe behaves very differently on small, quantum scales. Are grains of sand a particles? Yes, they are, but on smaller scales they consists of atoms, electrons, quarks - all of them being quantum objects. And those quantum properties - especially superposition - discovered unconsciously by Young in 1801 had put necessarry ground for current development of quantum computing. By the way -there is even a quantum field theory (QFT) that tries to holistically describe how our universe is functioning (theory of everything), combining Einsten's theory of relativity with quantum mechanims - but let's leave QFT for another episode.

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