New chip design for quantum computing

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.”

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

Originally published here.