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A 3-step guide to Quantum Mechanics

what are quantum mechanics, superposition and entanglement and why does it matter?

So what exactly is quantum mechanics, you might be wondering? An even more relevant question lingering in your head at this very moment might be – why should I actually care about quantum mechanics? Luckily, the answer to the latter is simpler: because quantum mechanics is at the basis of Quantum Computers. Quantum mechanics is what made Quantum Computers possible in the first place thanks to such quantum phenomena as superposition and entanglement.

The answer to the first question is however somewhat more complicated and it take years to fully grasp what quantum mechanics really are. However, in this brief post we’ve taken the challenging task of explaining in 3 brief steps the most important elements of quantum mechanics from a computing perspective.

Step 1: What is Quantum Mechanics?

Quantum mechanics (a.k.a. quantum physics or quantum theory) is a branch of physics that focuses on explaining the behaviour of particles on the level of an atom. Just like classical mechanics offers us a set of laws that describe the motion of bodies of a regular scale, quantum mechanics offers us laws relating to the motion of particles and their interactions with energy on a miniscule scale – the scale of an atom.

In other words, if you throw a ball in the air you know it will go up, reach a certain limit and then fall back down. But at the level of an atom the interactions between atoms, that physicists have observed, vary greatly from the familiar movements of objects that we see every day.

Quantum mechanics was pioneered by the likes of Albert Einstein, Max Planck, Erwin Schrödinger and Niels Bohr.

The 1927 Solvay Conference with Max Planck (bottom row, second from left), Marie Skłodowska-Curie (bottom row, third from left), Albert Einstein (bottom row, in the middle) and Niels Bohr (middle row, first from right), Erwin Schrödinger (top row, middle).

Their discoveries lead to a ground-breaking understanding of our world. The contributions of quantum mechanics had a great impact on our modern lives. After all, quantum theory has found its applications in many areas such as quantum chemistry, lasers, transistors, semiconductors, microprocessors, MRIs or cryptography. In the last 20 years principles of quantum mechanics have also proven useful in the design of a new breed of powerful computers – Quantum Computers. Those computers have the potential to reach previously unavailable levels of computing power and solve many currently unsolvable problems.

The functioning of Quantum Computers relies on two key quantum mechanics principles: superposition & entanglement.

Step 2: What is superposition?

In simple terms, superposition is a principle of quantum mechanics which states that when you are not looking at a given object (for instance an atom or an electron), that object can be in one or more states at the same time. In other words, a given electron can at the same time have two or more different positions, different temperatures, different colours different speeds etc. For instance, if an electron can be either black or white - than until we look at it to analyse its colour, the electron can be both black & white at the same time.

This simple explanation is however far from being simple to understand. In fact it doesn’t seem make much sense. If a car is black it is simply always black – before, while and after we look at it. If the car is parked in front of me, than it is simply parked in front of me. The car is not black and white at the same time – nor is it parked in two places at the same time. The world at a macro scale – the one we see on a daily basis – does not offer us much proof for such quantum theories. However, as mentioned earlier, the quantum world – the world of atoms and particles – behaves in accordance to very different rules.

What superposition means in fact is that until you saw something happen (a.k.a. until you measured the properties of a given particle), then it happened in all possible ways. If the particle can be in Place A or Place B, then it is in both Place A & B at the same time – until you look at it. In fact, as part of the superposition rule, physicists stated that whenever we measure the property of an object, our object has to choose just one state. Therefore if we measure the position of our particle, it will have to be either in Place A or in Place B during the measurement. It cannot be in both places during the measurement. It can however be at a different place during its subsequent measurement.

An example illustrating superposition has been provided in 1935 by Erwin Schrödinger. Schrödinger’s example was in fact intended to ridicule the existing view of quantum mechanics. Ironically, since then it has been proved that his example is quiet a real possibility of how superposition works.

Schrödinger imagined a cat closed in a box. The box contains also a flask of poison which has a 50% chance of being broken, hence poisoning the cat. The cat in this situation enters a state of superposition – it is both dead and alive. It is only when we open the box that we can see whether the cat is alive or dead.

Step 3: What is entanglement?

The second theory at the core of quantum mechanics is even more bizarre to comprehend than the first one. Entanglement is defined as a phenomenon in which two or more particles interact in such a way that the state of one cannot be described independently of the others – even when the particles are separated by great distances.

If two particles are in an entangled state than the measurement of one will determine the measurement of the other. If we imagine two electrons. Both can be either black or white. From superposition we know that we don’t know what colour each electron will have until we actually measure it. If our electrons are entangled, then the information about one of the them will serve us to obtain information about the other. If that is the case, whenever our first electron is black, we know the second one is black too. Knowledge about the colour of one electron gives us certainty in determining the colour of the other electron.

It remains somewhat unclear how this exactly happens. Some physicists claimed that particles “communicate” with each other at lightspeed – after one particle is measured it “sends a message” to its other entangled particle as to which state the latter should have. However this theory proved to be incorrect as experiments have shown that entanglement happens immediately and that nothing - not even at lightspeed can pass between the two particles.

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