What is Quantum Computing? - Science Simplified

January 05, 2024

Introduction

If you're interested in science, then perhaps you've been left wondering, what exactly is Quantum Computing? For the past century the field of quantum mechanics has enjoyed a notorious reputation in pop culture for being impossible to understand. Terms like quantum entanglement, quantum teleportation and Schrödinger's Cat are some of Hollywood's favourite terms to throw into the mix anytime weird sci-fi concepts need explaining. Thankfully, quantum mechanics - and by extension, quantum computing - are not in fact impossible for the average person to understand. While this blog post won't have you manically writing equations on a blackboard with frazzled grey hair, by the end of this article you should have a clearer picture about the emerging technologies of quantum computing, and what the future might hold for such an exciting industry. We also hope that this blog shows people that science doesn't have to be intimidating, in fact even the most complex of topics can be explained in a simpler way!

This article will be broken down into a few sections, starting with the history of quantum mechanics. Then we move on to the basics of some quantum phenomena that you need to feel familiar with, to understand why quantum computers have an inherent advantage over normal computers. Then quantum computers will be explained, and some of the possible uses are discussed. Finally, we wrap up by asking the question: "Will quantum computing take over the world?".

Here is an outline of the following sections of the article:

The History of Quantum Mechanics
Quantum Computing for Dummies
Quantum Computing Explained
Will Quantum Computing Take Over the World?

1. The History of Quantum Mechanics

What on Earth does Quantum actually mean?

Quantum mechanics had its inception in the late 19th century, while German physicist Max Planck was investigating a phenomenon known as black-body radiation. A black-body is an object that absorbs all the light that enters it, the most famous example of a black-body is of course a black hole. Planck introduced a mathematical theory to explain this black-body radiation which stated that energy emitted from a black-body could only be emitted in a quantised form [1]. Quantised is the key word here, and you might notice that it sounds a lot like quantum. If something is quantised it's not continuous, before quantum mechanics scientists believed that sub-atomic particles known as electrons orbited the nucleus of atoms everywhere. But this was shown not to be the case, in 1913 Niels Bohr developed the first model of a quantised Hydrogen atom [2], where electrons can only orbit the nucleus at certain energy levels. See the image below, the electrons are moving around the nucleus in different rings.

The Quantum Revolution of the Early 1900s

Now that we understand what it actually means to be quantum, we travel to the early 1900s, where you will meet some familiar household names!

It was not long after the work of Max Planck that everybody's favourite mad scientist produced the work that won him a Nobel prize! In 1905 Albert Einstein continued Planck's work [3] by explaining a phenomenon known as the photoelectric effect. All materials are made of atoms, and light is made of particles even smaller than atoms known as photons. When light hits a material, the photons penetrate the surface of the material and knock small particles off the atom, these small particles are known as electrons. If light is made up of particles called photons, then it might become clear to you that light is in fact quantised, because it's made up of lots of small building blocks (photons). The photoelectric effect is another example of the appearance of quantum mechanics when we look at very tiny particles.

It was in the 1920s that things really started heating up for quantum mechanics! French physicist Louis de Broglie released his work on wave-particle duality, stating that quantum particles behave both like quantised particles, and like waves [4]. Around the same time, another household name, Erwin Schrödinger, developed the famous Schrödinger wave equation, which explains the mathematics of how quantum particles behave like waves. At this point you're probably wondering how on earth something can be both a particle and a wave? We will explain this in the next section... stay tuned!

2. Quantum Computing for Dummies

To understand wave-particle duality, we first need to understand what waves are. When you think of waves, your mind probably travels to Spain, back to the beautiful beaches on your summer holiday, and that's perfectly correct. The waves in the Mediterranean exhibit the same properties as the mathematical waves in Schrödinger's equation. Waves oscillate (move up and down), and they have amplitudes (the height of the wave from the surface of the sea), they also have wavelengths (the distance between the peaks of the waves). The frequency of a wave is the inverse of the wavelength, and the frequency is directly related to the energy of the wave! Take a look at the diagram below to understand the structure of a wave, and its most important properties.

That's all fantastic, but that still doesn't explain why it's important to think of quantum particles as waves! In order to understand this, think of a tennis ball. If a scientist wanted to observe a tennis ball, they could simply look at it. If a scientist wanted to weigh a tennis ball, they could place it on some scales and write down the mass. If a scientist wanted to figure out how fast a tennis ball was moving, they could calculate the distance it travels over time. Quantum particles, however, cannot be seen by the human eye, they can't be picked up and placed on some scales, and their speed certainly can't be calculated with a pen and some paper! Or can it!?

While a quantum particle can't be seen by the naked eye, its behaviour can be modeled using the Schrödinger wave equation:

Schrodinger's equation - curiousminds.co.uk

Don't worry, we're not going to make you do maths! This equation looks complicated, but it actually describes a pretty simple idea. First let's talk about the horrible looking squiggly thing, this is called a wavefunction:

quantum wavefunction - curiousminds.co.uk

The wavefunction is simply a mathematical object that describes all of the properties of the quantum particle at any given time. Remember how we talked about the amplitude, wavelength, frequency, and energy. All of these properties are stored inside the wavefunction!

It's easy to see that the wavefunction appears three times within the Schrödinger equation. Once on the left of the equals sign, and twice on the right. The part of the Schrödinger equation on the left of the equals sign tells us about how the wavefunction changes over time. In other words, as time moves on how do the properties of the wave change? Maybe the energy of the wave increases or decreases, or maybe the position changes, or maybe everything changes! We can figure this out from the Schrödinger equation.

The part of the equation on the right of the equals sign tells us that the particle's wavefunction changes over time based on the kinetic and potential energies of the particle. In other words, the wavefunction of a particle is described by some very basic quantities that you learned about in high school physics class: mass, velocity, and potential energy. With these very simple quantities (and Schrödinger's equation), we can explain how a quantum particle behaves as a wave!

Quantum Superposition Explained

Now that we understand that quantum particles behave like particles, but also like waves, and that we can use equations to understand the wave-like behaviour. We can delve into some important concepts in quantum mechanics. The first of those is called quantum superposition.

Thankfully, superposition does not only exist in the quantum realm, superposition is a property of waves more so than a property of quantum mechanics. Nonetheless, it is vital to understand superposition to understand how quantum computers work. To draw a picture of superposition, let’s take things back to the ocean, when we are in the ocean, we can make waves with our hands by pushing water around, yes, we all did it as kids (and we still do as adults, don’t lie). But have you ever tried making waves with both hands and pushing them into each other? Do you remember what happened? When we do this, the two waves meet, and they either push each other up to form a bigger wave, or they cancel each other out. The result is known as a superposition of the two waves.

Remember how we talked about quantum particles being quantised, and how they can sit on different energy levels? Imagine we have an electron, and the electron has 3 different energy levels it can sit on. We don’t know for sure which energy level the electron is on until we have measured it in a lab. The wavefunction of the electron tells us the probability that the electron is sitting on each of the three energy levels. In other words, the wavefunction is in a superposition of the three energy levels! This is an extremely important concept for quantum computers.

3. Quantum Computers Explained

So here we are. You have learned all the tools you need to know to understand quantum computing! In this section we will explain how a quantum computer works, and some of the famous quantum computing algorithms that have been developed. We will also discuss the possible applications of quantum computing, and why you should be excited about the field!

Quantum Computers vs Classical Computers - Qubits Explained

Where normal computers use bits, quantum computers use qubits AKA quantum bits. Bits in a normal computer can either have the value 1 or 0, and sequences of these 1’s and 0’s form binary code, which is how humans can talk to computers and give them instructions. Qubits on the other hand can be more than just 1 or 0, do you remember how we talked about superposition, and how a quantum particle can have some probability of being in more than one different energy level at a given time? This is exactly why a qubit can be more than just 1 or 0! Because real life quantum computers are made up of quantum particles, each particle represents a qubit, and so each qubit can be in a superposition of many different energy levels.

Real quantum computers are made up of many individual quantum particles/qubits, and all of these qubits are in superpositions of every possible energy level possible. This means that together, all of these qubits can represent FAR more information than a computer with a bunch of normal bits. When a quantum computer works through a calculation, it can calculate every possible outcome simultaneously! This is known as quantum parallelism. Where classical computers would need many lifetimes to carry out certain calculations, qubits allow quantum computers to finish the calculations in a fraction of a second!

Applications of Quantum Computing

One such example of this quantum advantage was discovered in 1994 by American mathematician Peter Shor, the algorithm is aptly named ‘Shor’s Algorithm’. Essentially the algorithm is used to find prime factors of an integer, we won’t go into detail but if you’re interested you can read more here [5]. Prime factors are incredibly important in cyber security because many password encryption methods use the fact that it is incredibly difficult to find prime factors of integers on classical computers to keep your passwords safe. Fortunately, quantum computers haven’t reached the point where they are reliable enough to do this, and thankfully scientists are already working on quantum encryption algorithms which might keep your passwords safe in the future!

4. Will Quantum Computing Take Over the World?

Unfortunately for everyone who is excited by quantum computing, there are some huge problems that quantum computers currently face. Quantum particles are tricksy, and they don’t like being told what to do, meaning current quantum computers are very unstable. While quantum computers are not yet ready to take on the world, there has been a significant interest in recent times. The world’s biggest tech companies such as Google and IBM are already in a quantum arms race to see who can be the first to build stable and high-performance quantum computers. The next decade will be an extremely exciting one, if reliable quantum computers become commonplace, our world will change as we know it, and very quickly!