What does a quantum computer look like?

What does a quantum computer look like?

The photos you have probably seen show a hanging "chandelier": stacked round plates connected by rods and looping cables. That is the cooling system for a superconducting quantum computer, the kind built by companies such as IBM and Google. Each plate is colder than the one above it. At the very bottom sits the processor, a chip that IBM says is not much bigger than the chips in a laptop. In use, the chandelier is sealed inside a metal casing, so the finished machine looks like a large cylinder with racks of control electronics beside it.

Why so cold?

Qubits are extremely fragile. Heat, vibration and stray electrical noise disturb their quantum state, an effect called decoherence. IBM says superconducting processors must run about a hundred times colder than one degree above absolute zero, which is around 0.01 K. Outer space, warmed by the leftover glow of the Big Bang, is about 2.7 K (ESA), so the chip is roughly 270 times colder than deep space.

Other kinds of quantum computer

What is a qubit in quantum computing?

An ordinary bit is either 0 or 1. A qubit (quantum bit) can be 0, 1, or, in IBM's words, "a weighted combination of both simultaneously". This is called superposition. When you measure a qubit, you always get a plain 0 or 1, with chances set by those weights. Try it: this question asks what a measurement will show.

The state of one qubit is often drawn as a point on a ball called the Bloch sphere: 0 at the north pole, 1 at the south pole, and superpositions everywhere else. You can rotate a qubit around the Bloch sphere in the Quantum Computing course, which works as a qubit simulator in your browser.

Why qubits are powerful

Describing n qubits takes 2ⁿ numbers. 3 qubits need 2³ = 8; 10 qubits need 1,024; 50 qubits need about 1.13 × 10¹⁵. Qubits can also be entangled, so their results are linked. Quantum algorithms use interference to make wrong answers cancel out and right ones reinforce. This is why, for some problems, a quantum computer could be enormously faster. It isn't faster at everything, though: for everyday jobs like browsing or spreadsheets, an ordinary computer is better.

Is there a quantum computer you can use?

Yes, but not on your desk. Real quantum computers exist and are used for research, but they are huge, delicate and run by specialist teams. IBM notes that researchers reach quantum computers through cloud computing: you write a quantum program on your own computer, send it over the internet, and the results come back. Companies don't sell them like PCs, so there is no shop price; access is usually paid for by usage or through research partnerships. To learn the ideas, a simulator is the best place to start, since small circuits run perfectly well on an ordinary computer.

What quantum computers can't do (yet)

Today's machines are noisy: their qubits make errors, and correcting those errors takes many extra qubits. They won't replace laptops or phones, and they don't simply try every answer at once, a common myth. They are expected to help most with specific problems, such as simulating molecules for chemistry and materials, and some kinds of optimisation. In the long run, they could break widely used encryption, which is why new methods are being adopted now, as our guide to data encryption explains. Try this question on what quantum computers can and can't do:

Common misconceptions

• "A quantum computer tries every answer at once." Measurement gives one result; algorithms must use interference cleverly to make it the right one.
• "Quantum computers will replace normal computers." They are for particular problems, working alongside ordinary computers.
• "The chandelier is the computer." It is mostly cooling and wiring; the processor is a small chip at the bottom.
• "A qubit is both 0 and 1 in the everyday sense." It is a weighted combination, and measuring it gives just one.

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