The week in quantum computing: flip-flop qubits and the hunt for Majorana fermions

Australian researchers have been involved in not one but two breakthroughs in quantum computing this week, bringing us ever closer to completing what has been referred to as the space race of the 21st century.

Flip-flop qubits

On Wednesday, it was announced that UNSW engineers have invented a new architecture for quantum computing that promises to make the large-scale manufacture of quantum chips both cheaper and easier than previously thought possible. Their new chip design allows for a silicon quantum processor that can be scaled up without the precise placement of atoms required in other approaches, enabling quantum bits (qubits) — the basic unit of information in a quantum computer — to be placed hundreds of nanometres apart and still remain coupled.

Lead author Guilherme Tosi, a research fellow at the UNSW-based ARC Centre of Excellence for Quantum Computation and Communication Technology (CQC²T), developed the concept with CQC²T Program Manager Andrea Morello and co-authors Fahd Mohiyaddin, Vivien Schmitt and Stefanie Tenberg, along with collaborators Rajib Rahman and Gerhard Klimeck of Purdue University. Their work has been published in the journal Nature Communications.

In the single-atom qubit used by Morello’s team, and which Tosi’s new design applies, a silicon chip is covered with a layer of insulating silicon oxide, on top of which rests a pattern of metallic electrodes that operate at temperatures near absolute zero and in the presence of a very strong magnetic field. At the core is a phosphorus atom, from which Morello’s team has previously built two functional qubits using an electron and the nucleus of the atom.

Tosi’s breakthrough is the creation of an entirely new type of qubit, using both the nucleus and the electron. In this approach, a qubit ‘0’ state is defined when the spin of the electron is down and the nucleus spin is up, while the ‘1’ state is when the electron spin is up, and the nuclear spin is down.

“We call it the ‘flip-flop’ qubit,” said Tosi. “To operate this qubit, you need to pull the electron a little bit away from the nucleus, using the electrodes at the top. By doing so, you also create an electric dipole.”

“This is the crucial point,” added Morello. “These electric dipoles interact with each other over fairly large distances, a good fraction of a micron, or 1000 nm.

“This means we can now place the single-atom qubits much further apart than previously thought possible. So there is plenty of space to intersperse the key classical components such as interconnects, control electrodes and readout devices, while retaining the precise atom-like nature of the quantum bit.

“Crucially, this new qubit can be controlled using electric signals, instead of magnetic ones. Electric signals are significantly easier to distribute and localise within an electronic chip.”

Tosi said the design sidesteps the need to space qubits at a distance of only 10–20 nm apart, noting, “If they’re too close, or too far apart, the ‘entanglement’ between quantum bits — which is what makes quantum computers so special — doesn’t occur.”

“Our new silicon-based approach sits right at the sweet spot,” said Morello. “It’s easier to fabricate than atomic-scale devices, but still allows us to place a million qubits on a square millimetre.”

The hunt for Majorana fermions

One day later, another quantum-based paper was published in Nature Communications — this time serving as the latest confirmation of a strange quasiparticle at the heart of the next generation of quantum machines being pursued by University of Sydney and Microsoft Station Q engineers.

Dr Maja Cassidy, a co-author on the paper currently based on the University of Sydney, said researchers at Station Q Sydney are currently building the next generation of devices that will use quasiparticles known as Majorana fermions — fermions that are their own antiparticles — as the basis for quantum computers. While the first generation of quantum bits suffers from interference from electromagnetic ‘noise’, qubits using Majorana fermions will have their information encoded through their topology, or geometry. By braiding the Majoranas, quantum information is encoded in a way that is highly resistant to interference.

The only problem, Dr Cassidy said, is that when Majorana fermions were first shown to exist in 2012, there were many who doubted the evidence. A challenge to prove the findings was put to a research team led by Professor Leo Kouwenhoven, who now leads Microsoft’s Station Q in the Netherlands. The newly published paper meets an essential part of that challenge.

In essence, the paper proves that electrons on a one-dimensional semiconducting nanowire will have a quantum spin opposite to its momentum in a finite magnetic field. According to Dr Cassidy, “This information is consistent with previous reports observing Majorana fermions in these nanowires.”

The technology pursued by Station Q Sydney and its global partners at Microsoft will use these robust properties in their pursuit to build the world’s first practical quantum computers. Dr Cassidy added that the findings will also be useful in spintronic systems, where the quantum spin and not the charge is used for information in classical systems.

Image caption: Artist’s impression of a ‘flip-flop’ qubit in an entangled quantum state. Image credit: Tony Melov.