Ultra-pure silicon brings useful quantum computing a step closer

An international team has produced the world’s purest silicon, which could bring quantum computers much closer to reality.

“What we’ve been able to do is effectively create a critical ‘brick’ needed to construct a silicon-based quantum computer,” says University of Manchester professor Richard Curry, a co-supervisor of the project.

“It’s a crucial step to making a technology that has the potential to be transformative for humankind.”

The same quantum properties which make quantum computers such a tantalising prospect make them prone to inaccuracy.

Qubits that make up quantum computers are very sensitive to small changes in their environment, including temperature fluctuations. This is why current quantum computers require near-absolute zero temperatures to function. Even then, they can only do so coherently for a fraction of a second.

“Fragile quantum coherence means computing errors build up rapidly,” says University of Melbourne professor David Jamieson, co-supervisor of the project which is detailed in a paper published in Communication Materials.

“With robust coherence provided by our new technique, quantum computers could solve in hours or minutes some problems that would take conventional or ‘classical’ computers – even supercomputers – centuries,” Jamieson adds.

A problem in creating coherent silicon qubits has been purifying silicon.

Most naturally occurring silicon is the desired isotope silicon-28. But about 4.5% is silicon-29, which has an extra neutron in its nucleus, causing the silicon-29 atoms to act as tiny magnets, throwing the quantum coherence out.

By firing a beam of silicon-28 at a silicon chip, the physicists were able to effectively replace all silicon-29 atoms in the sample.

The process reduced silicon-29 in the chip to 0.0002% – 2 parts per million.

“The great news is to purify silicon to this level, we can now use a standard machine – an ion implanter – that you would find in any semiconductor fabrication lab, tuned to a specific configuration that we designed,” Jamieson says.

The current record for the longest single-qubit coherence of 30 seconds is held by the team at the University of Melbourne.

“Now that we can produce extremely pure silicon-28, our next step will be to demonstrate that we can sustain quantum coherence for many qubits simultaneously. A reliable quantum computer with just 30 qubits would exceed the power of today’s supercomputers for some applications,” Jamieson says.

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