The potential of ‘hot qubits’

Can we create warmer, cheaper and more robust quantum computing just using conventional silicon chip foundries?

Yes, suggests an Australian-led research team in a proof-of-concept paper in the journal Nature, which describes a way around one of the biggest constraints to quantum computers achieving their potential.

Currently most such computers will only work at fractions of a degree above absolute zero.

That requires multi-million-dollar refrigeration and, says research leader Andrew Dzurak from UNSW Sydney, as soon as you plug them into conventional electronic circuits they’ll instantly overheat.

However, a new quantum processor unit cell on a silicon chip – developed with collaborators in Canada, Finland and Japan – works at 1.5 Kelvin, which is 15 times warmer than competing chip-based technology being developed using 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,” says Dzurak.

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

Dzurak’s team announced initial experimental results in February last year, then in October a group in the Netherlands led by one of his former postdoctoral researchers unveiled similar “hot qubit” using the same UNSW technology. Game on.

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

The unit cell developed by 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 expensive cooling.

It also would be easier to integrate with conventional silicon chips, the researchers say, 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, which presents a challenge for designers.

“Every qubit pair added to the system increases the total heat generated,” says Dzurak, “and added heat leads to errors. That’s primarily why current designs need to be kept so close to absolute zero.”

The Royal Institution of Australia has an education resource based on this article. You can access it here.