Quantum Computing Explained Simply

Since I first heard this term years ago (probably at university), I've always been intrigued by what a "quantum computer" could actually be.

I know computer science (pretty well), I know classical physics, and a few concepts of quantum physics, but I didn't understand how the two could be linked to form a "machine" capable of performing calculations, and supposedly billions of times faster than classical computers.

That's why I decided to demystify the concept of quantum computing to present it to you in the simplest way possible.

The concept

The first thing that differentiates our classical computing from quantum computing lies in the scale of the elements used to perform calculations and store information.

To simplify, a classical computer uses a large quantity of copper atoms to channel moving electrons (electricity) to components that will use this electric charge to change state (like a switch), to store data and perform calculations using the properties of semiconductor materials like silicon.

Each silicon transistor can store 1 bit (1 or 0).

A quantum computer, on the other hand, uses individual neutral atoms (rubidium for example), which it moves and configures using lasers in order to store data and perform its calculations.

Each atom can store 1 qbit (or quantum-bit, the equivalent of 2 classical bits, superposed)

The difference

What is the point of using a quantum computer rather than a classical computer?

To understand the difference between the two, you need to note something very important: the order of magnitude.

In the classical system, millions of atoms of matter (copper, silicon, etc...) and billions of electrons interact with each other, whereas in the quantum system, the atoms are separated to be manipulated one by one.

Put this way, the difference doesn't seem huge, but you have to understand that when you're at the scale of the atom, that changes everything.

Indeed, thanks to classical physics, we're able to predict overall what a group of atoms, or electrons, will do. But individually, at the quantum scale, this becomes physically impossible because their behavior is unpredictable.

To understand this change of scale, imagine a car manufacturing plant powered by 2000 employees: You can predict that this plant will produce around 180 vehicles per day (give or take one), always of the same quality and the same characteristics.

Yet, at the scale of a single employee, you will never be able to predict that this employee will always provide the same work, at the same pace, at the same time. They might even get sick, or resign.

And yet the system as a whole will continue to function and remain predictable.

Well, there's the difference in scale between a quantum computer and a classical computer, and why their internal workings are different.

The principle of qBits

I told you that quantum computers could store the equivalent of two superposed bits in a single atom, but how does that work?

I could tell you that it's thanks to a quantum property called superposition of states, but that wouldn't get you very far. Just imagine that we replace atoms and electrons with balloons:

In a classical computer, we'd take a balloon to pull a switch (the transistor), so we could only measure a single state each time (on or off, 0 or 1), i.e. one bit.

In a quantum computer, we take a balloon (in place of our atom), and instead of throwing it, we spin it, or not. If it spins from top to bottom, then its first value will be 1, but if it spins from left to right, it's its second value that will be 1.

Which means that if we spin this balloon both left to right AND top to bottom (a final trajectory that will be diagonal), then we will have superposed two values, i.e. one qBit.

The possible values of a qBit are therefore: (0,0), (0,1), (1,0) and (1,1)

In this case, we don't actually spin an atom on itself, but the principle remains the same for the superposition of states.

What constraints?

Maintaining a "handful" of atoms configured in such a way as to represent data for performing calculations is extremely complex. In particular, it is necessary to keep them at a very low temperature in order to prevent them from moving.

Moreover, just configuring the grid of atoms (like a classical memory grid) is perilous and doesn't work every time.

This implies two major constraints:

• The number of simultaneous qBits in a quantum computer remains relatively low. (In 2021, the record is 256 qBits)
• Each calculation must be redone dozens or even hundreds of times to compensate for the lack of precision of current quantum processors

What calculations?

Currently, a quantum computer is capable of solving the same problems as a classical computer, but limited to the extent of its power measured in bits (i.e. 512 bits for the current record), which is quite little.

Nevertheless, quantum computing wasn't invented to solve the same problems, and the same algorithms, as a classical computer.

It is by using numerous, highly complex quantum properties (superposition of states, entanglement, unpredictability,...) that it becomes possible to solve certain algorithms that would be too complex for a classical system, because the internal workings of our electronics aren't designed for that.

One of the simplest examples to understand is, for instance, generating a random number, because in computer science, it is impossible to generate a purely unpredictable number, whereas in quantum computing, it is possible.

If you want to learn more, I wrote an article titled "Why randomness doesn't exist in computer science"

In summary

A quantum computer is different because it performs calculations with atoms selected one by one, which no longer obey classical (Newtonian) physics, but the complex laws of quantum physics.

We don't use quantum computers to perform the same calculations that our computers do, but to solve algorithms specifically tailored for them, and against which classical computing runs into trouble.

To go further

If the subject of quantum computers interests you, I invite you to listen to the excellent interview with Constantin Dalyac on IFTTD, himself a Quantum Software Engineer!