Ever read an article on quantum, even articles on this blog, and your eyes just glaze over at words like "qubit," "entanglement," "coherence," "superposition," or even the word "quantum" itself? When I first started learning about everything going on in the quantum industry, I had no idea what any of this meant. I didn't even know what a qubit was, which now seems so small in the larger scale of things. The point is, it is ok and honestly understandable to feel confused or overwhelmed with the terminology. A lot of them are explained through complicated chemical and physical processes that are only touched on in advanced college/graduate level classes. So, for those of you that are looking to develop a necessary quantum vocabulary to understand the articles you are reading, but don't feel the need to get a degree in quantum mechanics or something of the sort, this is for you. I hope this helps clear up some possible confusion and makes you feel more confident to take up space in different quantum conversations you may be having. Who knows, maybe this will be your launch pad into further quantum inspiration like me :)
What is Quantum?
Quantum refers to a branch of science that deals with really small things (particles, ions, atoms, photons, etc). We live in a world of big things (humans, buildings, dogs, you name it), and because of that we follow a set of laws. You may remember from a high school physics class being taught Newton's laws; "an object in motion remains in motion unless acted on by another force." These laws govern what is referred to as classical physics. When things get really small, down to a particle level, they follow a different set of laws. There is this graphic I really like from YouTuber Domain of Science
As you can see in the graphic, there is the world of classical physics which we are in, and some other things like relativity and philosophy. There is also the Chasm of Ignorance which I love because it shows that there are gaps in scientist's understanding of where all these worlds and their laws connect. It can be a little frustrating that the grey areas between these worlds can't be explained, but exciting that new discoveries are on their way. Now, back to the world of quantum physics and small things. Let's get you set up with some vocabulary that deals with these small things, the laws that govern them, their properties, and applications.
Types of Small Things
When I say quantum deals with small things what am I actually talking about? Here are a list of common small things that are used in quantum computing:
Atom: The smallest unit of a chemical element. Think a singular carbon atom.
Neutral Atom: A neutral atom is an atom with a net zero charge (same number of protons as it does electrons)
Ion: An ion is an atom that has a positive or negative charge, also known respectively as cations and anions. Common examples are Cl- or Na+
Photon: A particle of light. This particle is at the smallest energy level a particle of light can carry at a certain wavelength.
Wave-Particle Duality
Before diving further into vocabulary dealing specifically with quantum computing, it is important to clear up possible confusion on the terms "particle," and "wave," mostly because they can be used to describe the same thing. A property of particles is that they can also behave like waves, and waves can also behave like particles. If you are interested in the history of this discovery, you can read here about the double-slit experiment. This is important however because it means that when dealing with a photon for example, you cannot sum up its actions by following either the properties of a particle or of a wave. In short terms, it is both. Let's continue to see how this duality determines other principles.
Superposition
Superposition is a quantum property that allows a particle's quantum system to be in many energy states (wavelengths) at the same time. To explain superposition, I like to use the example of quarters. When you flip a quarter, it lands on either 0 or 1. Imagine 0 and 1 as two energy states a particle could be at, 0 the ground (resting) state or 1 the excited (added energy) state. The particle usually sits at either one of these states. However, the quantum property of superposition allows for the particle's quantum system to be at both states at the same time. Picture the quarter flipping in the air, it is both heads and tails at the same time. However when the quarter lands, and the observer views the side the quarter lands on, the superposition of the quarter collapses. This observation can be further explained through the idea of Schrödinger's cat.
Schrödinger's Cat: The Superposition Paradox
Schrödinger's cat refers to the idea that superposition can only exist when the particle is unobserved. Much like our quarter example, when an observer is involved to get a reading of the particle's state, the superposition collapses and it goes back to 0 or 1. In Schrödinger's cat, the thought experiment involves a cat in a box with a vial of poison that can or cannot be released based on a subatomic event. Meaning, when the cat is in the closed box, it is both alive and dead (in superposition). If someone were to open the box, the cat would be just alive OR dead, it cannot be both. In the quantum systems we will talk about below, it is important WHEN the superposition is collapsed through observance, so information can be successfully communicated and read.
Entanglement
As we discussed previously, quantum things act as both particles and waves. Waves can be understood through a mathematical model called a wave function. Entanglement refers to the principle that two quantum particles can be entangled so that they share the same wave function. This is a mathematical way to express the idea that these two particles are linked by sharing the same properties across any distance or barrier. When something happens to one particle, it happens to the other. My boss at P33, Meera, described entanglement to me in a way that I think is really helpful and I will share it with you. Think of two people standing on different sides of a wall. They cannot see one another. One of the people starts waving. The other person starts to wave back. If the people are entangled, the person on the other side of the wall can know that the person is waving to them despite not seeing them. This connection is a good metaphor for entanglement, and this connection is something only available to quantum things.
Constructive vs Deconstructive Interference
Back on the idea that quantum particles can also be waves, waves can interact with each other to form types of interferences. Constructive interference refers to when waves overlap in a productive manner. This comes from the waves being the same phase (1+1=2). Deconstructive interference refers to when waves overlap in a nonproductive manner. This comes from the waves being opposite phases (1+(-1)=0); they cancel each other out. These interactions of waves help determine how a quantum computer finds solutions to different problems. Constructive interference can help amplify the correct answer, whereas deconstructive interference can help cancel out the wrong answers.
Qubits
Now onto qubits, the basis of anything in quantum computing. Classical computers work in bits which are 0s and 1s. A qubit is a quantum bit (very creative with the naming here). In classical computers, bits are used to store information. The same thing is true with qubits, but their quantum properties allow them to carry the information differently. Qubits can be modeled with a Bloch sphere (pictures above). The special thing about a qubit is that it doesn't have to be just 1 or 0. It can be 1, 0, or any combination of those states in between. If you are a visual person like me, look at the arrow in the sphere pictured above. Say the pink dot at the top is 0 and the blue dot on the bottom is 1. The sphere is 3D and the arrow could be pointing in any direction (representing a combination of the states 0 and 1). Superposition is when a qubit is in this combination state. However, when it comes time to read out the qubit and the information inside, the superposition collapses into either 0 or 1. How to tell which one can depend on which direction (up or down) the arrow is pointing. These qubits participate in all of the properties described above. Entanglement is when two qubits are linked together to form one system. This property is what gives quantum computing its speedy potential. If you make a change to one of the qubits, their entangled counterpart will be changed as well. Check out some more examples of qubits and their entangled applications here in my articles on quantum ghost imaging and quantum teleportation. Qubits are also special for their properties of interference. In terms of computing, the constructive and deconstructive interference of wavelengths leads to the probability of getting the correct answer. This interference is a programming technique of the computer, using operators and gates to shift the systems probability in the right direction. In terms of the qubits, gates and operators can shift the qubits energy (wavelengths=energy), so that when the qubit's superposition collapses into 1 or 0, it is more probable to show the correct readout.
Coherence
Coherence describes how long a qubit can hold onto its information, something important to preform complicated calculations or transport information over a distance. Qubits are not the most stable things, the fact that they are in a state of superposition means they are unstable. Keeping them in a controlled and noise-free (noise in the sense of outside interference of any kind) is essential. This way a qubit can retain superposition and continue to carry its information. Decoherence is when the qubit collapses from superposition and into a classical state (0 or 1). It is important that the qubit is collapsed at a specific time, after certain operations and gates have been preformed to get the correct answer. Thus, decoherence is strongly unfavorable and there are many efforts being made to extend qubit life time and keep it free from environmental interference.
Quantum Computer
So what are these qubits being used to build? In some cases they are being used to create a quantum computing device. The term quantum computer may be confusing because the computers we are talking about do not look like the laptops or desktops we frequently use today. They can look something like the picture here on the left. There is no singular type of quantum computer that everyone is working to build. In fact, there are many that have been or are being created at a variety of scales and methodologies. The article earlier this week on Rigetti's new chip is an example of something one of these larger devices could use for computation. Many computing devices run on different types of qubits (photons, electrons, neutral atoms, etc). The type of qubit helps determine the hardware of the computer, leading to a variety of different software. Currently, there is no standardized quantum computer. We are in a period of great innovation, and there is a lot to look forward to when it comes to quantum discovery.
Further Learning
I hope this article has helped you in some way have a better grasp on the terms you see floating around in the quantum ecosystem. If this is something that has peaked your interest, I have collected some of the other resources I used to teach myself about quantum. Look out next week for an article detailing the different types of qubits and the process for building a quantum computer!
WIRED video on quantum principles explained on 5 different levels - great for an overview of quantum computing and how it can be discussed with people from different backgrounds and interests.
Quantum Computing for the Curious article - one of my favorite articles that gives a more detailed look at quantum computing in an engaging format.
The Map of Quantum Computing- similar to the map of physics pictured above, the video takes you through quantum computing principles and their applications.
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