Hot Qubits No More an Impediment to Quantum Computers?

For high paced computing that quantum computers are capable to carry out, its developers need multi-million-dollar refrigeration. Moreover, as soon as they plug them into conventional electronic circuits, it can result in instant overheating.

To curb this issue, researchers led by Professor Andrew Dzurak at UNSW Sydney have come up with new results to open a path from experimental devices to affordable quantum computers for real-world business and government applications.

As noted by the UNSW blog, the researchers' proof-of-concept quantum processor unit cell, on a silicon chip, 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' worths of refrigeration, rather than the millions of dollars needed to cool chips to 0.1 Kelvin," explains Prof. Andrew Dzurak.

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

Quantum computers are expected to outperform conventional ones for a range of important problems, from precision drug-making to search algorithms. Designing one that can be manufactured and operated in a real-world setting, however, represents a major technical challenge.

The UNSW researchers believe that they have overcome one of the hardest obstacles standing in the way of quantum computers becoming a reality.

In a paper published in the journal Nature, Dzurak's team, together with collaborators in Canada, Finland, and Japan, reported a proof-of-concept quantum processor unit cell that, unlike most designs being explored worldwide, doesn't need to operate at temperatures below one-tenth of one Kelvin.

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 multi-million-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 can perform the complex calculations needed to design new medicines, for example, will require millions of qubit pairs and is generally accepted to be at least a decade away. This need for millions of qubits presents a big challenge for designers.

"Every qubit pair added to the system increases the total heat generated," explains Dzurak, "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, expensive, and pushes refrigeration technology to the limit.

The UNSW team, however, has created an elegant solution to the problem, by initializing and "reading" the qubit pairs using electrons tunneling between the two quantum dots.

The proof-of-principle experiments were performed by Dr. Henry Yang from the UNSW team, who Dzurak describes as a "brilliant experimentalist".