Quantum hack brings computing closer

Australian team demonstrate 400% efficiency gain in critical qubit process. Andrew Masterson reports.

Quantum entanglement is spooky, but awfully fragile.
Quantum entanglement is spooky, but awfully fragile.
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Qubits are delicate little things, which makes the business of building a robust quantum computing system very difficult indeed.

But now, scientists from Australia’s University of Sydney have nutted out a way to dramatically improve their resilience, bringing the dream of a working real-world quantum computer much closer.

Qubits – or quantum bits – are the keystones of quantum computing. They can be understood as analogues for the binary “bits” used in classical computing. Like bits, they represent a basic unit of information. Bits, however, can only ever be in one of two possible states (1 or 0, say), while qubits can be in a superposition comprising both states at once.

When two qubits are entangled, the stage is set for what Einstein famously derided as “spooky action at a distance”. If a measurement is taken of one of a pair of entangled qubits, measuring the other one will yield exactly the same result – even if the second particle is thousands of kilometres away (or even, theoretically, at the other end of the universe).

It’s easy to see how this property is potentially very useful for conveying information, storing or encrypting information, which is one reason why researchers around the globe are very excited about quantum computing.

There is, however, a major practical problem: qubits are fragile and it’s very easy for them to be disrupted by all manner of things happening in their immediate environment. When this happens, they lose their quantum properties – they decohere, in the jargon – and with it, any information they encoded.

For scientists working in the field, qubits’ decoherence – arising often simply from the technology used to generate them in the first place – has been the devil in the detail.

Now, however, a paper in the journal Physical Review Letters outlines a theoretical advance that permits a 400% increase in the amount of interference a quantum computing system can endure before breaking down.

A team led by the university’s David Tuckett, Stephen Bartlett and Steven Flammia report a way of realising a significant increase in what is known as the quantum error correction threshold. This defines the level of noise generated by a real-world system below which quantum computing can still occur in an “ideal” state.

By modifying an element known as the surface code in their theoretical quantum system, the team found it could produce as “enormous gain” in the error correction threshold, lifting it to 43.7%. Tuckett and his colleagues call this breakthrough a “hack”, because it was achieved through re-working existing codes to assume realistic noise levels instead of a pristine theoretical state.

“This is achieved by tailoring our quantum decoder to match the properties of the noise experienced by the qubits,” says Flammia.

According to the researchers, the hack should be applicable to all quantum systems, regardless of whether they rely on superconductors, trapped ions, or semiconductors.

Having created theoretical proof of concept, the next stage of research will involve applying the approach to real-world, really noisy hardware.

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Andrew Masterson is news editor of Cosmos.
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