'Hot qubits' overcome quantum computing hurdle

Researchers from UNSW have developed a new proof of concept that is said to promise warmer, cheaper and more robust quantum computing — manufactured using conventional silicon chip foundries.

Quantum computers are expected to outperform conventional ones for a range of important problems, from precision drug-making to search algorithms. But designing a quantum computer that can be manufactured and operated in a real-world setting represents a major technical challenge, because most of those currently under development will only work at fractions of a degree above absolute zero. That requires multimillion-dollar refrigeration, and as soon as you plug them into conventional electronic circuits they’ll instantly overheat.

Now researchers led by UNSW’s Professor Andrew Dzurak have addressed this problem, overcoming what they believe to be one of the hardest obstacles standing in the way of quantum computers becoming a reality. Together with collaborators in Canada, Finland and Japan, the researchers created a proof-of-concept quantum processor unit cell, on a silicon chip, that works at 1.5 Kelvin — 15 times warmer than the main competing chip-based technology being developed by Google, IBM and others, which uses superconducting qubits.

“This is still very cold, but is a temperature that can be achieved using just a few thousand dollars’ worth of refrigeration, rather than the millions of dollars needed to cool chips to 0.1 Kelvin,” Prof Dzurak said.

“While difficult to appreciate using our everyday concepts of temperature, this increase is extreme in the quantum world.”

Prof Dzurak’s team first announced their experimental results in February last year. Then in October, a group in the Netherlands led by a former postdoctoral researcher in Prof Dzurak’s group, Menno Veldhorst, announced a similar result using the same silicon technology developed at UNSW in 2014. The confirmation of this ‘hot qubit’ behaviour by two groups on opposite sides of the world has led to the two papers being published ‘back to back’ in the same issue of Nature.

Cheaper and easier to integrate

Qubit pairs are the fundamental units of quantum computing. Like its classical computing analogue — the bit — each qubit characterises two states, a 0 or a 1, to create a binary code. Unlike a bit, however, it can manifest both states simultaneously, in what is known as a ‘superposition’.

The unit cell developed by Prof Dzurak’s team comprises two qubits confined in a pair of quantum dots embedded in silicon. The result, scaled up, can be manufactured using existing silicon chip factories, and would operate without the need for multimillion-dollar cooling. It would also be easier to integrate with conventional silicon chips, which will be needed to control the quantum processor.

A quantum computer that is able to perform the complex calculations needed to design new medicines, for example, will require millions of qubit pairs, presenting a big challenge for designers. Prof Dzurak explained, “Every qubit pair added to the system increases the total heat generated, and added heat leads to errors. That’s primarily why current designs need to be kept so close to absolute zero.”

The prospect of maintaining quantum computers with enough qubits to be useful at temperatures much colder than deep space is daunting and expensive, and pushes refrigeration technology to the limit. The UNSW team has created an elegant solution to the problem by initialising and ‘reading’ the qubit pairs using electrons tunnelling between the two quantum dots.

“Our new results open a path from experimental devices to affordable quantum computers for real-world business and government applications,” said Prof Dzurak.

Image caption: Dr Henry Yang and Professor Andrew Dzurak with a dilution refrigerator designed to keep qubits operating at extremely cold temperatures. Image credit: UNSW Sydney.

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