How does quantum computing work?

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23 January
10:14
30 January
13:27

Quantum computing involves using the weird properties of quantum mechanics to build a computer. We haven’t built one that works yet, but if a quantum computer is built it will be a very different computer than ones we’re using now.

When you think of how a computer like your laptop works, information is encoded in a series of zeros and ones – or bits. These bits correspond to different voltages in parts of the circuitry. When you try to process information, the processor inside your computer changes voltages very, very rapidly, changing zeros into ones and ones into zeros. A normal computer works by doing lots of individual operations extremely quickly.

A quantum computer is fundamentally different. One of the aspects of a quantum computer is that instead of having bits that may have a value of zero or one, information is encoded by quantum bits, which can have the value of both zero and one at the same time. Making a quantum bit is hard, but people have had some success with tiny particles of light, or with electrons around an atom.

The general idea is that you try to connect a bunch of these quantum bits together in a process called entanglement: if you imagine you had two entangled quantum bits, you could have the first in a state of zero or one, and the second in a state of zero or one. So the system together could be in 00, 10, 01, or 11 – four possibilities. In a quantum computer they’re actually in all of those states at the same time when you’re not observing it, and they ‘pick‘ one of these states to be in when you are observing. 

“A quantum computer would look at every possible solution to a problem, because it is can be in many configurations at the same time, and then settle on the right one.”

Now imagine you keep adding more and more quantum bits, then the system can be in a huge number of possible states. You can use this complexity to calculate extremely difficult problems that would be impossible with a normal computer. One of the challenges is trying to encode a problem in this system of quantum bits, say, asking it to crack a code or predict the weather. Once you’ve done that you can let these quantum bits evolve, and they will – hopefully – take the configuration that represents a solution to your problem. What has happened is that the quantum computer has looked at every possible solution to this problem because it is possible for it to be in many configurations at the same time, and settled on the right one, something a normal computer would take forever to do.

So that’s how it works in theory. The difficulties are finding materials to make these quantum bits. As I said, you could use electrons and atoms, you could use photons, little bits of light… but it’s challenging to get these things to become entangled and to stay that way. The most we can do now is stick three, four, maybe five of these quantum bits together.

The other challenge is: how do you get this computer to do something useful and then read information out of it? But people are making lots of progress on this, and it’s very interesting because it’s considered a potentially revolutionary technology. It takes problems that would take you billions of years to solve with even a super-computer, and makes them theoretically solvable in an instant.

That includes things like finding the factors of very large numbers, which is the basis of encryption technology right now. Whenever you do a transaction on the internet and you send your credit card details, the reason why people can’t intercept that information is that it is very hard to factor large numbers. People are looking at this quantum computing technology very, very closely, but the technological hurdles are still enormous. But who knows? 10 years, 20 years, 30 years, maybe these will be solved.

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