From the course: Quantum Computing Fundamentals

Classical bits vs. quantum bits

From the course: Quantum Computing Fundamentals

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Classical bits vs. quantum bits

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- You probably don't think about it as you're using a computer from day-to-day, but under the hood, that computer stores and processes all of its data using basic units of information called bits. A single bit by itself isn't very useful, but when you combine multiple bits together, they can be used to represent complex information. For example, this video you're watching is stored as a sequence of millions, perhaps billions of individual bits in a specific order that your computer interprets to produce what you're seeing and hearing. Now. In a classical computer like the one you're using to watch this, each of those individual bits can be in one of two possible states. We usually represent those states as one or zero, but they can also be described as true or false, on or off, yes or no. Those are all just abstract names that we use to describe one of two distinct states. Just like how you might describe the two sides of a coin as heads or tails. A coin laying on the table is like a classical bit because it will be in either one state or the other. - Quantum computers also process information as individual bits, but those quantum bits called qubits are special. In addition to being in one of two reliably distinguishable states, one or zero, a qubit can also exist in a state called super position. This is a principle of quantum mechanics that allows multiple states in a quantum system to be added together or superposed. The combination of these states means that a single quantum qubit can be both one and zero at the same time. Using Baron's coin example, we can represent a qubit in super position by giving coin a spin. While it's spinning on its edge, the coin is in a state that's not entirely heads or tails. In a sense, it's simultaneously both. Now, keep in mind this is an analogy that we're using to explain a concept. - This coin is not a quantum particle. It's just a coin. That means it doesn't follow the same rules, a quantum mechanics that a qubit does. A coin cannot really be in a state of superposition of heads or tails at the same time, and we're mentioning that here because we'll be using objects and analogies like this throughout the course. - As we do, keep in mind, these are teaching tools to relate the strange rules of quantum physics to the world as we experience it. - Speaking of teaching tools, here's another way to think about super position. Using an analogy we'll revisit throughout this course. Imagine that Olivia and I are having lunch at a quantum cafe. - This cafe has a very limited menu. You can only get a soup or a salad. Those are the two possible states. - I represent a classical bit because I always know exactly what I want. I'm having soup today. Now, some computational operation might change the value of my bit. - Maybe you should have the salad. It's healthier. - I'm definitely getting the salad. As you can see, my state can change, but as a classical bit, at any given moment, I'm fully committed to one option or the other. There's no in-between for me. - Unlike Baron's classical mindset, when I choose between two options, I think like a quantum bit. I can consider ordering the soup, and I can consider ordering the salad at the same time. I'm in a super position of both options with some probability of picking one over the other. - That just sounds indecisive. - Not indecisive. Superposed. I can't stay in super position forever, so I'll eventually have to settle on ordering one or the other, but until then, I can consider both. - The reason quantum bits can exist in super position, and classical bits can't is that, they're physically implemented in different ways. Qubits are stored using tiny particles that act as quantum mechanical systems with two physically distinguishable states. For example, in electron it can be used as a qubit because it has two distinct states referred to as spin up and spin down. A single photon could also be used as a qubit with the two states represented by vertical or horizontal polarization. Electrons and photons are both particles that display the peculiar effects of quantum mechanics such as super position. - The way cubits are physically implemented can have an impact when programming for real quantum hardware. However, this course isn't about the physics of quantum computers. We're going to look at the general concepts related to quantum programming and not focus on how a specific type of quantum computer physically works. - If you've ever written code for a classical computer, you probably didn't think about whether the bits you were manipulating, were being physically represented by the voltage on a wire. light in a fiber optic cable or magnetic fields on a hard disc. You just thought about those bits as being ones or zeros, usually to represent some higher level of information. That's the level of abstraction we'll use as we talk about qubits throughout this course.

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