(Image: D-Wave)
Baffled by how a quantum computer is supposed to work? Don’t worry, some of the biggest brains in physics can’t figure it out either. Some say such computers run in a swathe of parallel universes; others claim they transcend all normal notions of space and time. Whatever the truth, here’s a rundown of the basics.
Qubit:
Ordinary computers use “bits” to process information. The basic unit of quantum computing is the qubit. These are physical systems that can exist in two different states, so they can represent the 1s and 0s that make up the binary code computers run.
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A qubit might be an electron held in a magnetic field, or a photon that is polarised so its spin can be easily manipulated. Preparing qubits, as well as reading and writing to them, involves some hardcore hardware. Depending on your choice of technology, you might need a ruby laser, a non-linear crystal or even a pink diamond.
Superposition:
This is where the magic happens! The advantage a qubit has over a normal bit is that it can be put into a superposition state, being 0 and 1 at the same time. But this is tricky to pull off – any stray heat, electromagnetic noise or physical bump can knock it out again. So you’ll have to invest in some serious refrigeration, tinfoil shielding and tiptoe around your quantum computer, or invest in a state of the art vibration containment system.
Even then, you can only run the computer for a limited time before the superposition collapses. You’ll want to check out this “coherence time” carefully, as well as evaluating how many errors are created.
Entanglement:
Okay, we lied, this is where the magic really happens. Thanks to what Einstein termed spooky action at a distance, two subatomic particles can become inextricably interlinked, or entangled. The link lets you manipulate multiple qubits at once. That’s what makes quantum computers so kickass: just eight qubits, held in superposition and entangled, can simultaneously represent every number from 0 to 255, letting you carry out many operations at once.
So that’s the other thing to look for when weighing up a purchase: how many entangled qubits can your chosen machine manage at once? Don’t set your sights too high. At the moment, 14 is the record, achieved in 2012 by Rainer Blatt’s group at the University of Innsbruck in Austria.
Error correction:
Even normal computers make mistakes. Sometimes a bit can be buffeted by a voltage spike or a passing cosmic ray, changing it from a 0 to a 1, say. Processors deal with this by keeping copies, but this isn’t an option for qubits, thanks to a law called the no-cloning theorem.
Fortunately there are error correction algorithms to get around this. The drawback is that these need a lot of qubits, anything between 100 and 10,000 times as many as needed for the actual computation you’re trying to perform. Happily, our ability to assemble arrays of qubits for error correction has come on leaps and bounds. And error rates have been creeping downwards too. In June, IBM unveiled error correcting code that is well suited to the large arrays of qubits expected to outperform regular machines. Essentially we’re where we need to be to start building interesting quantum computers.
Read more: “Quantum computers: The world’s first buyers’ guide“
