Article 1 - Quantum Mechanics - The basis of Quantum Computing

The winter of 1947 saw the invention of perhaps the most useful device since the invention of the wheel, a transistor. The inventors, John Bardeen, William Shockley and Walter Brattain jointly won a Noble Prize in Physics in 1956 “for their researches on semiconductors and their discovery of the transistor effect”. This heralded the era of computing which led to where we are today, a modern society where the boundaries between humans and machines are blurring rapidly.

Now let's perform a little thought experiment. Imagine you are on your morning commute to work in Birmingham, UK, on a sunny (or more likely a cold, dull and rainy) Monday morning of May 29th, 1961. During this commute, you are reading your usual newspaper, The Birmingham Post, where amongst the various articles that talk about careers in Metallurgy, Electrical engineering, Technician, Craftsman and construction there is an article on a “different” career in electronic computers.

Newspaper snippet from May 1961 (Source - https://www.britishnewspaperarchive.co.uk)

Imagine that as a part of the general population in 1961 whilst you may have had heard of the thing called “electronic computers” you weren’t aware of (or had no way to know) how important it would become in the decades since. Would you have read the article and decided that you wanted to know more and get trained on computers or maybe someday switch jobs to take up a role that had to do something with computers? Chances are no, you would not. Because unless you were in a related field you wouldn’t take up computers as your career choice just because you wouldn’t have a clue of how impactful, important and highly paid the career choice could have been.

Fast forward to today and you are reading a similar article on Quantum Computing (QC) where you almost certainly know about the “electronic computers” or “classical computers” as it nowadays referred to in QC related articles. You may even be a programmer whose entire adulthood has been spent on building real-world applications on computers. You are today in the same position as your imaginary you were in 1961 but what is different this time is that you know how impactful computers have been since 1961 and hence you have the hindsight to make a judgement of how the invention of Quantum Computing would likely have an impact on the future of mankind. What I can say with certainty is that whatever impact you can imagine QC having on the future of mankind, your imagination is still not wild enough.

This is the first in the series of blogs where I will write about QC and related topics in a simple language with the aim that the readers can understand the potential this new technology brings with it and make a call themselves on how this new technology could shape the future of mankind.

I will begin in this first blog by talking about the fundamentals of Quantum Mechanics based on which the QC technology exists.

In physics, a quantum (plural: quanta) is the minimum amount of any physical entity (physical property) involved in an interaction. Quantum mechanics (QM; also known as quantum physics, quantum theory, the wave mechanical model, or matrix mechanics), including quantum field theory, is a fundamental theory in physics which describes nature at the smallest scales of energy levels of atoms and subatomic particles. Quantum mechanics differs from classical physics in that energy, momentum, angular momentum and other quantities of a bound system are restricted to discrete values (quantization), objects have characteristics of both particles and waves (wave-particle duality), and there are limits to the precision with which quantities can be measured (the uncertainty principle).

The above paragraph notes 3 different characteristics of QM and we must talk about all 3 very briefly to understand the basis of QC and why I am so excited about this game-changing technology.

The first is quantization which simply is a process by which we transition from our classical understanding of physical phenomena to a newer, more accurate understanding known as QM. Whilst it isn’t necessary for us to understand these characteristics in detail, it is fundamental for the engineers and physicists working on building a QC.

The next is wave-particle duality, which for me personally is mind-blowing. Ever heard of the "double-slit experiment"? If not, just go to YouTube and search for the experiment. There are a plethora of videos available for you to view but do make sure that you only view the ones that don’t go into the fringe theories and stick to the scientific explanation of the experiment.

In short, the experiment involves an electron (or photon would do too) that is fired from the electron gun and made to pass through a sheet that has 2 slits thus making sure that the electron could only go through either of the two slits hitting an observing screen behind it that lits up every time the electron is detected on the screen. As shown in the diagram below –

What transpires is completely counter-intuitive and frankly difficult to even believe, an interference pattern on the observing screen, even if you fire just one electron a time!

Now let’s fire up our imagination again and replace the quantum entity, electron, with something that we all must have seen, a bowling ball. Make that ball a freshly painted one with the paint still wet so as to ensure it leaves a mark when it hits anything. Now imagine throwing the bowling ball towards a wall with 2 gaps (like the double-slit) and finally hitting an “observing wall” behind it. Something like what the images show below -

Freshly painted bowling ball thrown at the wall with 2 gaps.

You could never imagine seeing an interference pattern developing on the observing wall because the bowling ball, just like our other everyday objects we all know and love, doesn't behave that way (Why, pray tell, doesn't it? you may ask. There is a good explanation for it and the Planck's constant has got something to do with it which I will leave for another day another article). The bowling ball either goes through one gap or the other and finally hitting the observing wall behind. Hence what you would see are the paint marks from balls that hit the observing wall directly behind the gaps of the previous wall (and hands with a lot of wet paint!), all other balls not going through the gaps would be deflected and never let past the wall with 2 gaps.

A real-world, imaginable, pattern that could develop when the balls pass through either of the gaps and hit the observing wall behind leaving the paint marks on it.

The interference pattern that could develop if the bowling ball was a quantum particle.

So how do you explain the bizarre behaviour of the quantum world electrons where effectively a single electron can be at more places than once all at the same time bumping into itself? It turns out that there is an explanation (otherwise this series of blogs would have had to stop here) but you will need to then remove the notion that the electrons are particles and replace them with waves, i.e. every electron behaves like a wave and the single-fired electron being at every possible location at the same time with a probability associated with where you could find the electron at any location at any given time. It's important to re-read the previous sentence as that is what is fundamentally different between quantum mechanics and classical physics. The diagram below gives a pictorial representation of the wave-like behaviour in the double-slit experiment where an electron, or a light source such as a photon, is fired one by one and the interference pattern that is visible due to the electron, or a photon, interfering with itself. Yes, you read that right, INTERFERING WITH ITSELF!

Mind blown yet? If not, then what is even more bizarre is when you put a detector just next to each of the gaps in the double-slit experiment to detect the actual path electron takes to reach the observing screen, it changes the behaviour of the electron. In that, the electron goes from being a wave to being a particle (wait what??) thus changing the interference pattern in the observing screen to what you would imagine a bowling ball does. Now if this doesn't excite you and bother you both in equal measure then either I haven't explained it well enough or you haven't understood what I have just explained.

Perhaps this snippet of a video may help those who find it hard to imagine what I have just described.

This change in behaviour of the electron after adding the detector to the experiment is called the “Observer effect” which means that the mere observation of a phenomenon inevitably changes that phenomenon.

This brings us to the third characteristics of QM, the uncertainty principle or also known as Heisenberg's uncertainty principle which is any of a variety of mathematical inequalities[1] asserting a fundamental limit to the precision with which certain pairs of physical properties of a particle, known as complementary variables or canonically conjugate variables such as position x and momentum p, can be known or, depending on interpretation, to what extent such conjugate properties maintain their approximate meaning, as the mathematical framework of quantum physics does not support the notion of simultaneously well-defined conjugate properties expressed by a single value.

If the above statement is too technical, which it is, then in simple language it means that you can either know the position of the quantum particle or the momentum, never both. That is a fundamental nature of the QM and Einstein did not like it one bit hence his famous quote “Quantum theory yields much, but it hardly brings us close to the Old One’s secrets. I, in any case, am convinced He does not play dice with the universe.” Einstein believed that if you knew everything that there is to know about a particular problem then you could, with 100% accuracy identify the various parameters of the solution. Let's take a flipping of a coin for instance. If you know all the information that impacts the flipping of a coin (and I mean everything, even as insignificant as the exact location on earth where it was flicked), i.e. the force at which it was flicked, the height from the ground at which it was flicked, the wind speed, the surrounding temperature, the weight of the coin, the atmospheric pressure around the coin, the make and material of the coin, gravity pull exerted on the coin by the earth, the moon and the sun at that time, etc. you can with 100% precision predict the side on which the coin would land. But in QM, the uncertainty is fundamentally ingrained in nature (which leads to other wonderful topics such as true random number generation which I will touch base on in future blogs).

Richard Feynman, one of the biggest proponents of QM and QC in recent times and a Nobel prize winner once quoted “I think I can safely say that nobody understands quantum mechanics”. That effectively sums it up.

Before I close this first blog, I would like to leave you with another piece of a quote from Richard Feynman that sets the stage for my next few blogs on QC – “trying to find a computer simulation of physics seems to me to be an excellent program to follow out. . . .the real use of it would be with quantum mechanics. . . . Nature isn’t classical . . . and if you want to make a simulation of Nature, you’d better make it quantum mechanical, and by golly it’s a wonderful problem because it doesn’t look so easy.”

A wonderful problem indeed!

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