Artificial atoms create stable qubits

Quantum engineers in Australia have created artificial atoms in silicon chips which, they say, offer improved stability for quantum computing.

And, they add, there are parallels to chemistry’s Periodic Table.

The idea of creating artificial atoms that have electrons but not a nucleus is not new, says Andrew Dzurak from UNSW Sydney. It was first proposed theoretically in the 1930s and then experimentally demonstrated in the 1990s.  

“But what really excites us about our latest research,” he says, “is that artificial atoms with a higher number of electrons turn out to be much more robust qubits than previously thought possible, meaning they can be reliably used for calculations in quantum computers. 

“This is significant because qubits based on just one electron can be very unreliable.”

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A silicon qubit high-frequency measurement stage, which is positioned inside a dilution refrigerator to cool the chip (the rectangle) to around 0.1 degrees above absolute zero. Credit: UNSW Sydney

Writing in the journal Nature Communications, Dzurak and colleagues describe configuring a quantum device in silicon to test the stability of electrons in artificial atoms.

They applied a voltage to the silicon via a metal surface “gate” electrode to attract spare electrons from the silicon to form the quantum dot, an infinitesimally small space of only around 10 nanometres in diameter.

“As we slowly increased the voltage, we would draw in new electrons, one after another, to form an artificial atom in our quantum dot,” says Andre Saraiva, who led the theoretical analysis of the results.

“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. 

“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.”

The researchers were interested in what happened when an extra electron began to populate a new outer shell – and that’s where comparisons to the Periodic Table come in.

In chemistry, the elements with just one electron in their outer shells include Hydrogen and the metals Lithium, Sodium and Potassium.

“When we create the equivalent of Hydrogen, Lithium and Sodium in the quantum dot, we are basically able to use that lone electron on the outer shell as a qubit,” says lead author Ross Leon. 

“Up until now, imperfections in silicon devices at the atomic level have disrupted the way qubits behave, leading to unreliable operation and errors. 

“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.”

And that is a crucial step towards silicon-based quantum computers becoming a reality, says Dzurak, because the spin of an electron is used to encode the value of the qubit.

“Spin is a quantum mechanical property,” he says. “An electron acts like a tiny magnet and depending on which way it spins its north pole can either point up or down, corresponding to a 1 or a 0.

“When the electrons in either a real atom or our artificial atoms form a complete shell, they align their poles in opposite directions so that the total spin of the system is zero, making them useless as a qubit. 

“But when we add one more electron to start a new shell, this extra electron has a spin that we can now use as a qubit again.”

The latest research, Dzurak says, shows that the team can control the spin of electrons in the outer shells of artificial atoms to provide reliable and stable qubits. 

“This is really important because it means we can now work with much less fragile qubits. One electron is a very fragile thing. However, an artificial atom with five electrons, or 13 electrons, is much more robust.”

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