Quantum computing: Cheat Sheet

How superpositions and spooky action at a distance could help factor massive numbers...

...quantum computer needs to be perfectly isolated from its environment until it has completed all its calculations. Attempting to read the data before a calculation has run its course will destroy the computer's ability to compute.

Enough of the science already, I feel a migraine coming on - tell me more about the technology. Has anybody made a quantum computer yet?
Not really - assuming you mean a fully fledged and capable machine consisting of hundreds or thousands of entangled qubits. So-called 'toy quantum computers' - with just a handful of qubits, say up to around 10 - have been made in labs and even made to perform basic calculations.

Scalability - building a quantum computer with scores of entangled qubits and therefore creating a really powerful machine - is arguably the biggest challenge for quantum computing. As mentioned previously, another massive challenge is overcoming the fragility of quantum states: any interference or noise from the environment, such as vibrations or stray particles, can collapse the quantum state. This so-called 'decoherence' is basically the quantum computer's equivalent of a blue screen of death.

So assuming scientists can sort all this tricky stuff out, what would a quantum computer be able to do? Presumably I'll be able to ditch the desktop and upgrade to a nice new shiny machine?
Not necessarily. In fact, the average computer user probably won't have much use for a quantum computer because these machines are not suited to all tasks: a quantum computer runs probabilistic algorithms and generates probabilistic results - so it's not going to improve your word processing or make Facebook more useful.

Probability has its uses, though. Such a system would be very useful in determining the factors of extremely large numbers - one of the hard maths problems that classical computers struggle with as they have to check every possibility one by one, which quickly becomes untenable as the time required to perform such linear calculations can run to scores of years.

However, by using a quantum computer you could quickly generate a relatively small set of probable factors which could then be checked with a classical computer to confirm the answer.

Quantum computers could help solve certain types of mathematical problems

Quantum computers could help solve certain types of mathematical problems
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Factoring massive numbers is what internet and banking security depends on, so security, financial services and military applications for quantum computers are easy to envisage.

Quantum computers - even small ones with tens of qubits - would also be better at simulating quantum systems than classical computers are, making even a series of toy quantum machines useful to physicists and other scientists seeking to build quantum systems or model complex interactions between quantum systems. Similarly, they could have...