Researchers Draw Closer to Developing Silicon-Based Quantum Computers

The innovative Australian method of making quantum bits from precisely positioned individual atoms in silicon is earning huge rewards, with UNSW Sydney-led scientists demonstrating for the first time that they can engineer two of these atom qubits to “talk” to each other.

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The team – headed by UNSW Scientia Professor Michelle Simmons, Director of the Centre of Excellence for Quantum Computation and Communication Technology, or CQC²T – is the only group in the world with the ability to view the precise position of their qubits in the solid state.

Simmons’ team makes the atom qubits by precisely positioning and encapsulating separate phosphorus atoms within a silicon chip. Information is stored on the quantum spin of an individual phosphorus electron.

The team’s recent advance – the first observation of well-regulated interactions between two of these qubits – is published in the Nature Communications journal. It follows two other latest breakthroughs using this unique method to constructing a quantum computer.

By enhancing their nano-manufacturing process, Simmons’ team has also in recent times developed quantum circuitry that has the lowest recorded electrical noise of any semiconductor device.

In addition, they have developed an electron spin qubit with the longest lifetime ever recorded in a nano-electric device – 30 seconds.

“The combined results from these three research papers confirm the extremely promising prospects for building multi-qubit systems using our atom qubits,” says Simmons.

2018 Australian of the Year inspired by Richard Feynman

Simmons, who was bestowed the honor of 2018 Australian of the Year in January for her pioneering quantum computing study, says her team’s pioneering work is inspired by the late physicist Richard Feynman.

“Feynman said: ‘What I cannot create, I do not understand’. We are enacting that strategy systematically, from the ground up, atom by atom,” says Simmons.

“In placing our phosphorus atoms in the silicon to make a qubit, we have demonstrated that we can use a scanning probe to directly measure the atom’s wave function, which tells us its exact physical location in the chip. We are the only group in the world who can actually see where our qubits are.”

“Our competitive advantage is that we can put our high-quality qubit where we want it in the chip, see what we’ve made, and then measure how it behaves. We can add another qubit nearby and see how the two wave functions interact. And then we can start to generate replicas of the devices we have created,” she says.

For the new research, the team positioned two qubits – one made of a single phosphorus atom and one made of two phosphorus atoms – 16 nm apart in a silicon chip.

Using electrodes that were patterned onto the chip with similar precision techniques, we were able to control the interactions between these two neighboring qubits, so the quantum spins of their electrons became correlated.

Dr. Matthew Broome, Lead Co-Author

“It was fascinating to watch. When the spin of one electron is pointing up, the other points down, and vice versa.”

“This is a major milestone for the technology. These type of spin correlations are the precursor to the entangled states that are necessary for a quantum computer to function and carry out complex calculations,” he says.

Study lead co-author, UNSW’s Sam Gorman, says: “Theory had predicted the two qubits would need to be placed 20 nanometres apart to see this correlation effect. But we found it occurs at only 16 nanometres apart.

“In our quantum world, this is a very big difference,” he says. “It is also brilliant, as an experimentalist, to be challenging the theory.”

UNSW Sydney-led scientists have shown for the first time that they can make two precisely placed phosphorous atom qubits “talk” to each other. (Video credit: UNSW)

Leading the race to build a quantum computer in silicon

UNSW scientists and engineers at CQC²T are racing ahead of the world to develop a quantum computer in silicon. They are developing parallel patented methods using single atom and quantum dot qubits.

“Our hope is that both approaches will work well. That would be terrific for Australia,” says Simmons.

The UNSW team has selected to work in silicon as it is among the most stable and easily manufactured environments wherein to host qubits, and its long history of application in the conventional computer industry means there is an enormous body of knowledge regarding this material.

In 2012, Simmons’ team, who employ scanning tunneling microscopes to place the individual phosphorus atoms in silicon and then molecular beam epitaxy to encapsulate them, produced the world’s narrowest conducting wires, measuring only four phosphorus atoms across and one atom high.

In a recent paper published in the Nano Letters journal, they used similar atomic scale control methods to create circuitry approximately 2-10 nm wide and showed it had the lowest recorded electrical noise of any semiconductor circuitry. This research was carried out jointly with Saquib Shamim and Arindam Ghosh of the Indian Institute of Science.

“It’s widely accepted that electrical noise from the circuitry that controls the qubits will be a critical factor in limiting their performance,” says Simmons.

“Our results confirm that silicon is an optimal choice because its use avoids the problem most other devices face of having a mix of different materials, including dielectrics and surface metals, that can be the source of and amplify electrical noise.”

“With our precision approach we’ve achieved what we believe is the lowest electrical noise level possible for an electronic nano-device in silicon - three orders of magnitude lower than even using carbon nanotubes,” she says.

Simmons’ team revealed, in another recent paper in Science Advances, their precision qubits in silicon could be made in such a manner that the electron spin had a record lifetime of 30 seconds – up to 16 times longer than formerly reported. The first author, Dr. Thomas Watson, was at UNSW undertaking his Ph.D. and is currently at Delft University of Technology.

“This is a hot topic of research,” says Simmons. “The lifetime of the electron spin – before it starts to decay, for example, from spin up to spin down – is vital. The longer the lifetime, the longer we can store information in its quantum state.”

In the same paper, they reported that these long lifetimes helped them to read out the electron spins of two qubits in sequence with an accuracy of 99.8% for each, which is the level needed for practical error rectification in a quantum processor.

Australia’s first quantum computing company

While a conventional computer performed calculations one after another, a quantum computer would function in parallel and be able to look at all the possible conclusions at the same time. It can solve problems within minutes which would otherwise take several years.

In the previous year, Australia’s first quantum computing company – aided by a unique consortium of governments, universities, and industry – was founded to commercialize CQC²T’s world-leading research.

Working out of new laboratories at UNSW, Silicon Quantum Computing Pty Ltd has the target of fabricating a 10-qubit demonstration device in silicon by 2022, as the forerunner to a silicon-based quantum computer.

The Australian government has financed $26 million in the $83 million venture through its National Innovation and Science Agenda, with an extra $25 million provided by UNSW, $14 million from the Commonwealth Bank of Australia, $10 million from Telstra, and $8.7 million from the NSW Government.

It is projected that industries covering approximately 40% of Australia’s existing economy could be considerably impacted by quantum computing. Probable applications include machine learning, software design, scheduling and logistical planning, stock market modeling, financial analysis, software and hardware verification, rapid drug design and testing, climate modeling, and early disease detection and prevention.