Quantum Computing: you know it’s cool, now find out how it works

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You have probably heard all the buzzwords people use when trying to explain quantum computing (superposition and entanglement ring a bell?). Fans of xkcd – “the webcomic of romance, sarcasm, math, and language” – already know that when a subject is both philosophically exciting and mathematically complex, it’s easy to develop weird ideas about it, like quantum. So, in this post, some of my colleagues and I thought we’d help clear up some common confusion about what these buzzwords mean, and how quantum computing actually works.

Let’s start with superposition. Actually, superposition itself is something we see every day in our classical world. Imagine playing two notes on a guitar; the sound you hear when you do this is a superposition of the two notes. Quantum superpositions are also made up of a combination of states, except the key difference is what happens when you perform a measurement. Despite the system existing in a perfectly well-defined superposition state, when you perform certain measurements on these systems you get random outcomes. So, the magic is actually observed as a special kind of quantum randomness. My colleague and member of the IBM Q team, Antonio Corcoles-Gonzalez, explains it better than I can:

Classical and Quantum Randomness

Now for entanglement. It’s this idea that you can’t describe two entangled particles independently of each other. Their states are tied together in ways that can’t be recreated in our classical world. If I measure one of them, I might observe that it behaved randomly, but it tells me what to expect when measuring the other particle, in the same way. As IBM researcher Kristan Temme explains in the video below, this phenomenon of perfect correlation holds true even if you measure your entangled particles at opposite ends of the galaxy. Harnessing entanglement for computation is considered to be a crucial ingredient for speeding up computation using quantum computers.

Quantum Entanglement

So how do quantum computers compute?

IBM researcher David Gosset’s explanation, below, is the best I’ve seen so far. Just like with classical computing, you need a set of instructions that represent a problem-solving approach (i.e. an algorithm), and you need a machine that can execute those instructions (i.e. a computer). The fact that quantum computers can actually create superpositions, entanglement, and other quantum effects means we get to write algorithms in a new way that we couldn’t before with classical machines.

Quantum Algorithms

Finally, how do you go from knowing what you want to do (i.e. create superposition, entanglement, etc.) to actually doing it on a real quantum computer through the IBM Q experience?

You use quantum gates, or operations that change the states of the qubits. IBM scientist Sarah Sheldon is a leading expert on quantum gates, so take it from her. In the video below, she’s picked out three examples to get you started with a quantum computer right away. After you check out her video, have a look at our Beginner’s Guide for a deeper dive.

Quantum Gates

As IBM Fellow, and father of quantum information theory Charlie Bennett says: “Get ready to think outside a box you didn’t know existed.” By now, you probably know what he means!

Director, Cloud Infrastructure Research, IBM Research

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