Quantum computing demystified

Time machines - oh, boy!
Steady on Sam, I love science fiction as much as the next geek but I'm not talking about Quantum Leap here. This is even more exciting than time travel.
OK, so what is this quantum computing lark then?
Quantum computing and quantum information processing are research efforts that seek to exploit quantum mechanical phenomena to perform tasks such as massively parallel computing. The quantum research field also encompasses quantum cryptography, which utilises quantum phenomena to guarantee secure communications.
What are these quantum phenomena you talk of?
Tsk! Clearly weren't paying attention in physics class were you?
Quantum mechanics is a branch of physics that describes all manner of weirdness and 'spooky behaviour' - that is, quantum phenomena - taking place at the atomic and sub-atomic levels where electrons, protons and other particles exist.
The quantum world's spooky behaviour, for example, sees matter and energy able to behave both like particles and waves simultaneously, and apparently exist in two places at once.

Quantum mechanics becomes evident at the atomic and sub-atomic levels
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My head hurts.
That's to be expected. All of this quantum weirdness is deeply counter-intuitive - if not downright bizarre - to our human brains because it stands in stark contrast to the classical physics we experience in our everyday lives.
Thanks for the physics refresher - but what does all this spooky behaviour have to do with computers?
Good question. Instead of having bits, as a classical computer does, which represent either a one or a zero, a quantum computer has quantum bits - qubits - which can represent zero, one, or a superposition of both - that is, any amount of either zero and one simultaneously. As a result, unlike a traditional computer which can only store one number in a single register at any one time, a quantum computer can store more than one.
Adding more qubits exponentially increases the size of the number that can be stored - a computer with a 100 qubits would be able to store a massive number in its register, for instance.
As well as qubits, another key element of quantum computing is a phenomena known as entanglement.
Hang on, what's quantum entanglement when it's at home?
I was afraid you were going to ask. Quantum entanglement is the point where scientists typically abandon all hope of being understood because the thing being described really does defy the classical logic we're used to.
An object is said to become quantumly entangled when its state cannot be described without also referring to the state of another object or objects, because they have become intrinsically linked, or correlated.
No physical link is required however - entanglement can occur between objects that are separated in space, even miles apart - prompting Albert Einstein to famously dub it "spooky action at a distance".
The correlation between entangled objects might mean that if the spin state of two electrons is entangled, their spin states will be opposites - one will be up, one down. Entangled photons could also share opposing polarisation of their waveforms - one being horizontal, the other vertical, say. This shared state means that a change applied to one entangled object is instantly reflected by its correlated fellows - hence the massive parallel potential of a quantum computer.
With enough entangled qubits at its disposal, a quantum computer then becomes a vehicle for doing massive parallel processing or tackling hard mathematical problems such as factoring huge numbers - a task that classical computers struggle with.
Once an entangled state has been created between two objects, it is also possible to know the other object's state indirectly by measuring its entangled fellow.
It's a bit like taking a peek at someone's ankle and checking out one of their socks - once you've seen the repeating sheep pattern poking out of their shoe, you can be pretty sure you know what's on their other foot. In the quantum world, meanwhile, you can be absolutely sure of the state of one entangled object after peeking at its correlated fellow.

Quantum entanglement - a bit like the correlation between a pair of socks
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OK I kind of get the idea but what is it used for? What sort of practical applications does it have?
Entanglement has various applications and potential uses. Quantum cryptography, for instance, uses the phenomena to guarantee secure communication. Should an eavesdropper interact with an entangled object - that is, by intercepting and listening to it - it would alter its entangled fellow and thereby give away the spy's presence.
As a result, if the entangled object arrives unchanged at its destination, it's possible to know with absolute certainty that a communication has not been intercepted en route.
Another process that makes use of entanglement is quantum teleportation, which essentially enables information stored on a qubit to be transferred from one quantum system to another without physically transporting the qubit itself or broadcasting its contents (neither of which is physically possible).
Teleportation of the data, however, can be achieved between a sender and a receiver with a little sleight of hand by utilising the correlation between a pair of entangled qubits that the two parties share between them to calculate and then recreate the information at the destination point.
And, as mentioned above, entanglement is also essential to a quantum computer - one scientist describes it as the fuel powering such a machine.
So how can entanglement be created?
Another great question, albeit one requiring a good head for science. Suffice to say, there are various ways of generating entanglement, just as different particles, atoms or physical facets can be used to act as qubits - such as protons, trapped ions, nuclear spins and so on.
Some objects also entangle more easily than others - photons, for instance, have been relatively easy for scientists to entangle while electrons have posed more of a challenge.

Lasers can be used to generate entanglement
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Pairs of entangled electrons can exist in cooled superconductors but in order to make use of them scientists have had to devise ways to separate and extract them from the material. This has been achieved by utilising another quantum phenomena known as quantum tunnelling - whereby a particle is able to defy classical physics and pass (or tunnel) through a high energy barrier - and then deploying tiny components such as nanocrystals or carbon nanotubes to act as gates and ensure the paths of the entangled electrons diverge.
Creating reliable entanglement on demand is crucial to creating a viable quantum computer and one of the problems scientists continue to wrestle with.
OK, these quantum computers need qubits and entanglement to work but what kind of hardware would they be made of?
Quantum computers are a new approach to data processing that exploit quantum mechanical phenomena but there is no agreed single way to build such a machine - scientists are in the process of trying various approaches, so it's difficult to know which will prevail in the long run.
Some of the methods being researched include using ions in traps to act as qubits which are entangled with lasers. Then there's optical systems using photons by targeting lasers on beam-splitters to generate qubits. There are also solid state quantum computing research efforts utilising artificial atoms known as quantum dots, or seeking to harness nuclear magnetic resonance to manifest quantum information. Electron-based systems in low temperature superconductors are another big research effort. There is also interest in the potential of carbon-based nanomaterials such as graphene to house electron-based qubits. And that's by no means an exhaustive list of the ongoing research.
Much effort is being expended on working out which technologies have the most potential to build a scalable quantum computer that also maintains its quantum coherence for long enough to perform the necessary calculations and give up the resulting data.
Since measuring a quantum state collapses it to a classical state then a 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
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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 applications in the pharmaceutical industry to model complex molecules. In essence quantum machines would thrive in situations that involves lots of variables - and where, therefore, classical computers soon run out of steam.

Quantum technology could have applications in the pharmaceutical industry
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So quantum computers don't replace classical computers, they would be used in conjunction with them to help solve particular types of hard mathematical problems?
Exactly. Quantum computers have lots of limitations which require a classical computer to step in and help out. Even when it comes to computational complexity theory they won't necessarily excel as there are some mathematical problems that might simply be unsolvable with any type of computer.
Anything else I should know?
One more thing, there is a minority of scientists who believe that building a quantum computer will turn out to be out-and-out impossible.
However, if those scientists are right, the implication of not being able to build such a machine is that quantum mechanics itself, as a description of nature, is wrong. Either way, the stakes could not be higher.
Alright, I'm intrigued. So when are we going to get one of these quantum computers - assuming we are indeed able to build them?
Another good question and one which scientists typically laugh at when asked to answer it. The short answer is that no one knows for sure - but it's certainly not going to be any time soon.
Jason Smith of Oxford University, a lecturer in the materials department and tutorial fellow at Mansfield College, said: "I think I can be fairly confident in saying we're not going to have anything which is going to be doing what you would consider to be a useful computation in the next 10 years."
Another scientist, 35-year-old Dave Bacon, assistant research professor at the University of Washington, told silicon.com he hopes to see a viable quantum computer existing outside a lab "in my lifetime" - so at the very least this technology looks likely to be decades more in the making.
I better hang on to my laptop then.
Yep, this is one upgrade that's not going to be pushing your tech kit into an early grave.