UNSW researchers have measured the accuracy of two qubit operations in silicon for the first time, giving fidelity results that "confirm the promise" of silicon as a viable and scalable platform for future quantum computers.
Professor Andrew Dzurak’s team at UNSW Engineering was the first to build a quantum logic gate in silicon - making calculations between two qubits of information possible - in 2015.
The achievement has been emulated by groups around the world, but an accurate assessment of the gates' fidelities had proven difficult, meaning their true accuracy remained an unknown.
In a paper published in Nature today, Dzurak and his team demonstrate the successful measurement of fidelity in a silicon two qubit gate using a method called 'Clifford-based randomised benchmarking'.
"To assess the accuracy of a qubit system we need to simulate how that system behaves for any possible calculation that a quantum computer is tasked to do. For calculations between just two qubits there are already over 11000 possible basic operations that can be performed. Each one produces a specific rotation of one spin dependent on the state of the other," said Dzurak.
These basic operations are called Clifford gates. To assess the accuracy of a two qubit operation, the researchers applied a huge, random set of Clifford gates to the two spins of their two-qubits and "see how close our two qubit state is to what we intended".
The average two qubit gate fidelity was found to be 98 per cent.
"That shows we're closing in on an accuracy that's close enough for quantum error correction," Dzurak said.
The experiments were performed by Wister Huang, a final-year PhD student in electrical engineering, and Dr Henry Yang, a senior research fellow at UNSW.
“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,” Huang, the lead author on the paper, said.
In April, Yang co-authored a paper with researchers at the University of Sydney which laid out how they achieved the record for the world’s most accurate one-qubit gate in a silicon quantum dot, with a fidelity of 99.96 per cent
“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,” Yang explained.
The results are "further proof" that the researchers' punt on silicon as a platform for scaling up to the large numbers of qubits needed for universal quantum computing is the right choice, they said.
“If our fidelity value had been too low, it would have meant serious problems for the future of silicon quantum computing. The fact that it is near 99 per cent 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,” Dzurak said.
Given the fidelities measured were limited by the relatively long gate times used compared with the decoherence times of the qubits, it is expected silicon qubit designs that employ faster gate operations, together with advanced pulsing techniques, will push the accuracy read-outs even higher.
“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," Dzurak said.
Dzurak is a project lead at Silicon Quantum Computing, Australia’s first quantum computing hardware company, which is hoping to commercialise spin qubits based on silicon CMOS (complementary metal–oxide–semiconductor) technology.
The other authors of the 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 the isotopically enriched silicon wafers needed for the project.
“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,” added UNSW Dean of Engineering, Professor Mark Hoffman.