Introduction to Quantum Computing
Here, inside this refrigerator, at a temperature just a tick above absolute zero, isolated from the rest of the universe... ...is a quantum computer. If you believe the hype, this nascent technology embodies the promise of the future and has the potential to revolutionise our lives with its turbo-charged computation. But quantum computers aren’t the next generation of supercomputers—they’re something else entirely. And before we can even begin to talk about their potential applications, we need to understand the fundamental physics that drives the theory of quantum computing. We’ll need to dive into another dimension, smaller and more alien than anything we intuitively understand: the subatomic world of quantum mechanics.
In the 1980s, one of the most important physicists of the 20th century encountered a major roadblock. Richard Feynman was hungry for a window into the quantum universe. But quantum systems, by nature, are fragile, and the information they hold hides from us. Because Feynman couldn’t directly observe quantum events, he wanted to design a simulation. It quickly became clear that his computer wasn’t up to the task. As he added particles to the quantum systems he was modelling, the cost of computation began to rise exponentially. Feynman concluded that classical computers just can't scale up fast enough to keep pace with the growing complexity of quantum calculations. Then he had a breakthrough. What if he could design a tool made up of quantum elements itself? This instrument would operate according to the laws of quantum physics, making it the perfect way to probe the mysteries of the quantum realm. The idea of the quantum computer was born. And by dreaming it up, Feynman had started to build a bridge between quantum physics and computer science.
How to quantum computers work
Superposition and Quantum entanglement
Physicists call normal computers as ‘classical’ computers, and classical computers work with binary. There’s loads of bits that can either be in a state of zero or one. In a quantum computer you have quantum bits, qubits, which can be in the state zero, or one, or they can be in a special intermediate state and that’s called a superposition. You might wonder what superposition is, well it’s a special phenomenon in quantum physics that quantum computers take advantage of. Superposition is the ability of a quantum system to be in multiple states at the same time until it is measured. This concept might be intuitively challenging so it is often portrayed by the famous schrodinger's cat thought experiment. Now when you measure a qubit the result you get is based on a probability, so if you set this superposition state to be in the middle, you’ve got a 50% chance of getting a zero and a 50% chance of getting a one. But you can tune that state to make it more likely to get a zero than a one, or the other way around. So that’s the first phenomenon of quantum physics that quantum computers take advantage of. The second one is called entanglement. Entanglement is another weird property of quantum particles. To get an idea of how quantum entanglement works, consider this simple example. Suppose you have a pair of gloves, and you place two of them in two different boxes. After that, you shuffle the boxes and ask your friend to open one of the boxes. If he opens the box and finds a right-handed glove, then without opening the second box, you know that it contains a left-handed glove. This will remain true even if you take the second box to the Moon or anywhere in the universe. That's because the two gloves were entangled entities. They had a connection between them: one was right-handed, and the other was left-handed.
Quantum entanglement is where you bring several different qubits and join them together, and now that whole thing has to be treated as one object. So if you take two qubits and join them together, this object can be in the mixed state of four states: 01
10 11 00
Each time you add a new qubit, you double the number of states that this thing can be in, and that goes up exponentially. So if you want to search through different states, a classical computer has to search through them one by one. But in a quantum computer you’ve got these special quantum algorithms where you can enhance the probability of the state that you want, and diminish the probability of it giving you back the state that you don’t want. So those two phenomena: entanglement and superposition are what give quantum computers their power.
An example showing how Quantum computers take advantage of the uncertainty principle would be the typical probability game of predicting head and tails. So imagine a game you play on a classical computer. It starts with a coin showing heads, and the computer will play first. It can choose to flip the coin or not, but you don't get to see the outcome. Next, it's your turn. You can also choose to flip the coin or not, and your move will not be revealed to your opponent, the computer. Finally, the computer plays again, and can flip the coin or not, and after these three rounds, the coin is revealed, and if it is heads, the computer wins, if it's tails, you win. Now on a classical computer, you have a 50-50 chance of winning the game, but you will get obscure results when you play this game on Quantum computers. On quantum computers, you can choose to edit the probability, for example you can choose to make it 100-0 or 40-60, depending on your choice. It might seem like magic or cheating, but actually, it's just quantum physics in action. Here's how it works:
A regular computer simulates heads or tails of a coin as a bit, a zero or a one, or a current flipping on and off inside your computer chip. A quantum computer is completely different. A quantum bit has a more fluid, nonbinary identity. It can exist in a superposition, or a combination of zero and one, with some probability of being zero and some probability of being one. In other words, its identity is on a spectrum. For example, it could have a 70 percent chance of being zero and a 30 percent chance of being one or 80-20 or 60-40. The possibilities are endless. The key idea here is that we have to give up on precise values of zero and one and allow for some uncertainty. So during the game, the quantum computer creates this fluid combination of heads and tails, zero and one, so that no matter what the player does, flip or no flip, the superposition remains intact. It's kind of like stirring a mixture of two fluids. Whether or not you stir, the fluids remain in a mixture, but in its final move, the quantum computer can unmix the zero and one, perfectly recovering heads so that you lose every time.
So what is the current state of development in this field?
There are a whole bunch of different companies trying to make a quantum computer: Google, Intel, IBM, Microsoft, D-Wave, amongst others. And to complicate matters, there’s not just one kind of quantum computer there’s actually a whole load of different approaches.
An important thing to point out here are Universal quantum computers. Universal quantum computing can theoretically simulate any quantum system and so it’s the fundamental computing machinery of the Universe. There’s other approaches that aren’t Universal like quantum annealing and ion trap systems. What they’re focused on are solving certain nice problems better than classical computers by taking advantage of quantum physics and they’re a valid approach if those kinds of problems are valuable. They can be used as a good stepping stone towards a Universal quantum computer, because Universal quantum computers are very difficult to build, and it’s good to learn stuff along the way. The above image also includes the number of qubits in these quantum computers as of today.
I find quantum computing exciting as a way to explore physics. Now, whether that's going to make anybody any money—whether there'll be practical applications in the near-term—that's still very much an open question. But at least for physicists, it's an exciting time. The truth is... that the most important application, I believe, of quantum computers is something that we don't know yet. I'm sure that once we have a quantum computer to play with, we'll find amazing applications that we can't yet foresee.