Quantum computing stability granted by ‘artificial atoms’

Using artificial atoms researchers could engineer stable qubits that could finally take quantum computing from theory to reality.

One of the major hindrances in making quantum computing a reality is the issue of stability. It takes very little environmental interference to disturb a quantum state and therefore cause a catastrophic loss of information. Achieving both stability and the effective control of electrons is vital in the development of useable quantum computers and quantum networks.

In a paper published in Nature Communications, quantum engineers from UNSW Sydney detail the creation of artificial atoms in silicon ‘quantum dots’ that could offer improved stability. Within these tiny spaces, electrons are used as qubits — the fundamental units of quantum information. Qubits are the quantum computing equivalent of bits in classical computing, but whereas a bit stores information as a binary ‘0’ or ‘1’ — a qubit can store values of 0 and 1 simultaneously as a result of quantum phenomena in which particles can exist in a superposition of states.

The most famous analogy for this is the Schrodinger’s Cat thought experiment —in which the unfortunate moggy is held in a superposition of being both dead and alive. This results in a vast increase in the number of calculations that can be conducted at one time — simultaneously rather than consecutively.

Quantum computers are able to conduct more calculations simultaneously due to the phenomenon of superposition (IBM)

The electrons in an artificial atom buzz around the centre of the device much like a traditional atom, but its centre is distinguishable by the lack of a nucleus. “ The idea of creating artificial atoms using electrons is not new, in fact, it was first proposed theoretically in the 1930s and then experimentally demonstrated in the 1990s — although not in silicon,” says Professor Dzurak, an ARC Laureate Fellow and director of the Australian National Fabrication Facility at UNSW, where the quantum dot device was manufactured. “We first made a rudimentary version of it in silicon back in 2013, but what really excites us about our latest research is that artificial atoms with a higher number of electrons turn out to be much more robust qubits than previously thought possible.”

More electrons mean more reliability in carrying out calculations in quantum computers. “This is significant because qubits based on just one electron can be very unreliable,” Dzurak explains.

The periodic table for quantum bits

In order to consider the best and most effective arrangement of electrons for quantum computing and information storage, Dzurak and his team constructed a rudimentary ‘periodic table’ for qubits.

He explains: “If you think back to your high school science class, you may remember a dusty chart hanging on the wall that listed all the known elements in the order of how many electrons they had, starting with Hydrogen with one electron, Helium with two, Lithium with three and so on.

“You may even remember that as each atom gets heavier, with more and more electrons, they organise into different levels of orbit, known as ‘shells’.

“It turns out that when we create artificial atoms in our quantum circuits, they also have well organised and predictable shells of electrons, just like natural atoms in the periodic table do.”

To test the stability of electrons in artificial atoms, the team applied a voltage to the silicon via a metal surface ‘gate’ electrode. This attracts spare electrons from the silicon to form the quantum dot — an infinitesimally small space of with a diameter of about 10 nm or 0.000001 cm. “As we slowly increased the voltage, we would draw in new electrons, one after another, to form an artificial atom in our quantum dot,” explains Dr Andre Saraiva who led the effort of theoretically analyse the team’s 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.”

As a result, rather than forming a sphere, the electrons are arranged in a flat disc-shaped pattern.

Valence electrons are the electrons that sit in an outer shell determining the chemical bonding processes that an element undergoes. (Science ABC)

The researchers wanted to know what happens when an extra electron was forced into a new outer shell. In the periodic table, the natural elements with just one electron in the outer shell — known as a valence electron — are Hydrogen (H), Lithium (Li), and Sodium (Na).

“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 Ross Leon, lead author of the paper. “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 as a ‘primer’ on the imperfect surface of the quantum dot, smoothing things out and giving stability to the electron in the outer shell.”

It's all about the spin

As Professor Dzurak explains, storing information with electrons acting as qubits is all about controlling a quantum mechanical property of particles known as spin: “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 team’s research demonstrates that the spin of these valance electrons in artificial atoms can be controlled, giving reliable and stable qubits for the storage of quantum information. And the more electrons available the better.

“Our new work shows that we can control the spin of electrons in the outer shells of these artificial atoms to give us reliable and stable qubits,” Dzurak says. “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 5 electrons, or 13 electrons, is much more robust.”

The team will now expand upon their research by investigating how the rules of chemical bonding, determined by the number of valance electrons possessed by an element, affect their artificial atoms. This knowledge could help the team construct artificial molecules which can then be used to create multi-qubit logic gates. Such devices would represent a major step on the road to the development of a large-scale silicon-based quantum computer.