# Researchers reveal `results `

of two-qubit calculations‘highly promising’ for quantum computing

## In what represents a world-first, researchers have measured the fidelity — the accuracy — of two-qubit logic operations in silicon — leading to highly promising results that will enable scaling up to a full-scale quantum processor.

The research was carried out by Professor Andrew Dzurak’s team in UNSW Engineering, with experiments performed by Wister Huang — a final-year PhD student in Electrical Engineering, and Dr Henry Yang, a senior research fellow at UNSW.

Dzurak explains: “All quantum computations can be made up of one-qubit operations and two-qubit operations — they’re the central building blocks of quantum computing.

“Once you’ve got those, you can perform any computation you want — but the accuracy of both operations needs to be very high.”

Dzurak’s team was the first to build a quantum logic gate in silicon back in 2015, making calculations between two qubits of information possible —the process clearing a crucial hurdle in the quest to make silicon quantum computers a reality.

Since then, a number of groups around the world have demonstrated two-qubit gates in silicon — but until this landmark paper published today in *Nature — *the true accuracy of such a two-qubit gate was unknown.

#### Accuracy is the key to quantum computing success

Dr Yang adds: “Fidelity is a critical parameter which determines how viable a qubit technology is — you can only tap into the tremendous power of quantum computing if the qubit operations are near perfect, with only tiny errors allowed.”

In this study, the team implemented and performed Clifford-based fidelity benchmarking — a technique that can assess qubit accuracy across all technology platforms — demonstrating an average two-qubit gate fidelity of 98%.

Mr Huang, the lead author on the paper, explains how such high accuracy was achieved: “We achieved such a high fidelity by characterising and mitigating primary error sources, thus improving gate fidelities to the point where randomised benchmarking sequences of significant length — more than 50 gate operations — could be performed on our two-qubit device.”

Quantum computers will have a wide range of important applications in the future as a result of their capacity to perform complex calculations at greater speeds than even the most sophisticated modern computer. This includes solving problems that are simply beyond the ability of today’s computers.

Professor Dzurak says: “But for most of those important applications, millions of qubits will be needed, and you’re going to have to correct quantum errors, even when they’re small.

“For error correction to be possible, the qubits themselves have to be very accurate in the first place — so it’s crucial to assess their fidelity.”

He continues: “The more accurate your qubits, the fewer you need — and therefore, the sooner we can ramp up the engineering and manufacturing to realise a full-scale quantum computer.”

#### Silicon is the way forward

The researchers say the study is further proof that silicon as a technology platform is ideal for scaling up to the large numbers of qubits needed for universal quantum computing.

Given that silicon has been at the heart of the global computer industry for almost 60 years, its properties are already well understood and existing silicon chip production facilities can readily adapt to the technology.

Dzurak says that if the fidelity value had been too low, it would have had serious negative implications for the future of silicon quantum computing.

He continues: “The fact that it is near 99% puts it in the ballpark we need, and there are excellent prospects for further improvement. Our results immediately show, as we predicted, that silicon is a viable platform for full-scale quantum computing.

“We think that we’ll achieve significantly higher fidelities in the near future, opening the path to full- scale, fault-tolerant quantum computation. We’re now on the verge of a two-qubit accuracy that’s high enough for quantum error correction.”

In a second paper authored by Dr Yang and recently published in *Nature Electronics, *the same team also achieved the record for the world’s most accurate 1-qubit gate in a silicon quantum dot — with a remarkable fidelity of 99.96%.

Dzurak commends the work of the UNSW team: “Besides the natural advantages of silicon qubits, one key reason we’ve been able to achieve such impressive results is because of the fantastic team we have here at UNSW.

“My student Wister and Dr Yang are both incredibly talented. They personally conceived the complex protocols required for this benchmarking experiment.”

UNSW Dean of Engineering, Professor Mark Hoffman, says the breakthrough is yet another piece of proof that this world-leading team are in the process of taking quantum computing across the threshold from the theoretical to the real.

Hoffman says: “Quantum computing is this century’s space race — and Sydney is leading the charge.

“This milestone is another step towards realising a large-scale quantum computer — and it reinforces the fact that silicon is an extremely attractive approach that we believe will get UNSW there first.”

#### Spin qubits

Spin qubits based on silicon CMOS technology — the specific method developed by Professor Dzurak’s group — hold great promise for quantum computing because of their long coherence times and the potential to leverage existing integrated circuit technology to manufacture the large numbers of qubits needed for practical applications.

Professor Dzurak leads a project to advance silicon CMOS qubit technology with Silicon Quantum Computing, Australia’s first quantum computing company.

Dzurak says: “Our latest result brings us closer to commercialising this technology — my group is all about building a quantum chip that can be used for real-world applications.”

A full-scale quantum processor would have major applications in the finance, security and healthcare sectors — it would help identify and develop new medicines by greatly accelerating the computer-aided design of pharmaceutical compounds. It could contribute to developing new, lighter and stronger materials spanning consumer electronics to aircraft, and faster information searching through large databases.

Other authors on the *Nature* paper are UNSW researchers Tuomo Tanttu, Ross Leon, Fay Hudson, Andrea Morello and Arne Laucht, as well as former Dzurak team members Kok Wai Chan, Bas Hensen, Michael Fogarty and Jason Hwang. Professor Kohei Itoh from Japan’s Keio University provided isotopically enriched silicon wafers for the project.

*Original research: **http://dx.doi.org/10.1038/s41586-019-1197-0*