UNSW use flat electron shells from artificial atoms as qubits

Artificial atoms comprised of shells of electrons, without a nucleus present, have been used in silicon as the building blocks for quantum computers.


Image: UNSW

Researchers at the University of New South Wales (UNSW), alongside those from the Université de Sherbrooke in Quebec and Aalto University in Finland, have announced they have used artificial atoms in silicon as quantum bits (qubits), in a paper published in Nature.

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The artificial atoms have electron shells just like actual atoms, but are without a central nucleus with a positive charge. Instead, the UNSW team use an electrode to provide the positive charge, with the electrons provided by the silicon into an area that is 10 nanometres in diameter dubbed the quantum dot.

"In a real atom, you have a positive charge in the middle, being the nucleus, and then the negatively charged electrons are held around it in three-dimensional orbits," Dr Andre Saraiva said.

"In our case, rather than the positive nucleus, the positive charge comes from the gate electrode which is separated from the silicon by an insulating barrier of silicon oxide, and then the electrons are suspended underneath it, each orbiting around the centre of the quantum dot. But rather than forming a sphere, they are arranged flat, in a disc."

In quantum computing, qubit value is stored in the spin of an electron, and with extra electrons being added to the artificial atom, this resulted in the formation of complete shells where the spin values cancelled each other out. This led to the research team becoming interested in artificial atoms that have one electron in the outer shell -- the artificial equivalent of hydrogen, lithium, or sodium -- which would allow for that outer electron to act and be measured as a qubit.

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"Up until now, imperfections in silicon devices at the atomic level have disrupted the way qubits behave, leading to unreliable operation and errors," PhD student and lead author of the paper Ross Leon said.

"But it seems that the extra electrons in the inner shells act like a 'primer' on the imperfect surface of the quantum dot, smoothing things out and giving stability to the electron in the outer shell."

The team added that compared to a single electron acting as a qubit, a qubit with five or 13 electrons would be much more robust.

Funding for the research was provided by the Australian Research Council, the US Army Research Office, the Australian National Fabrication Facility, and the National Science Engineering Research Council of Canada.

The researchers will next move onto how chemical bonding works with the artificial atoms to create artificial molecules that could be used to create better multi-qubit logic gates.

Among the research team is Professor Andrew Dzurak, who was involved at UNSW in measuring the accuracy of two-qubit logic operations in May 2019.

"All quantum computations can be made up of one-qubit operations and two-qubit operations -- they're the central building blocks of quantum computing," Dzurak said at the time. "Once you've got those, you can perform any computation you want -- but the accuracy of both operations needs to be very high."

Researchers from the university unlocked the key to enabling quantum computer coding in silicon in late 2015, announcing that UNSW at the time had built a quantum logic gate in silicon, making calculations between two qubits of information possible.

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