Phosphorous qubit milestone reported

Australian researchers notch up third advance in quantum computing. Andrew Masterson reports.

Quantum computers are now one step closer.
Quantum computers are now one step closer.
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Australian scientists are claiming a milestone in quantum computing after successfully demonstrating that individual phosphorus atoms positioned in a silicon substrate can be made to “talk” to each other.

In a paper published in the journal Nature Communications the researchers show communication between qubits – the essential units of quantum information – created by precisely siting phosphorous atoms within a silicon chip, with information stored in the atom’s spin.

By employing a technique called scanning tunnelling microscopy lithography, the scientists are able to directly measure each atom’s wavefunction, thereby determining its exact location in the chip.

“We are the only group in the world who can actually see where our qubits are,” explains team leader Michelle Simmons of the University of New South Wales.

“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.”

The achievement comes hot on the heels of two other major accomplishments led by Simmons, who was recently named the 2018 Australian of the Year.

She and her colleagues created quantum circuitry with the lowest recorded electrical noise of any known semiconductor device. They also reported an electron spin qubit with a lifetime of 30 seconds – 16 times longer than anything previously achieved in a nano-electronic device.

“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 their latest work, Simmons’ team created two qubits – one comprising two phosphorous atoms and the other using just one – and placed them 16 nanometres apart. Using electrodes patterned into the silicon chip, the scientists were able to control interactions between the two qubits, such that the quantum spins of their electrons became correlated.

“It was fascinating to watch. When the spin of one electron is pointing up, the other points down, and vice versa,” says lead co-author Matthew Broome.

“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.”

Even the distance between the qubits marks a significant advance.

“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,” explains co-author Sam Gorman.

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

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Andrew Masterson is news editor of Cosmos.
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