Engineers discover new way to control spins in a quantum dot

Engineers from quantum computing startup Diraq and UNSW Sydney have discovered a new way to control single electrons nestled in quantum dots that run logic gates. The new mechanism is less bulky and requires fewer parts, which could prove essential in making large-scale silicon quantum computers a reality. The discovery is detailed in the journal Nature Nanotechnology.

“This was a completely new effect we’d never seen before, which we didn’t quite understand at first. But it quickly became clear that this was a powerful new way of controlling spins in a quantum dot. And that was super exciting,” said Dr Will Gilbert, lead author and a quantum processor engineer at Diraq.

Logic gates are the basic building block of all computation; they allow ‘bits’ — or binary digits (0s and 1s) — to work together to process information. However, a quantum bit (or qubit) exists in both of these states at once, a condition known as ‘superposition’. This allows a multitude of computation strategies — some exponentially faster, some operating simultaneously — that are beyond classical computers. Qubits are made up of ‘quantum dots’, tiny nanodevices which can trap one or a few electrons. Precise control of the electrons is necessary for computation to occur.

While experimenting with different geometrical combinations of devices approximately billionths of a metre in size that control quantum dots, along with various types of minuscule magnets and antennas that drive their operations, Dr Tuomo Tanttu came across a strange effect.

“I was trying to really accurately operate a two-qubit gate, iterating through a lot of different devices, slightly different geometries, different materials stacks and different control techniques. Then this strange peak popped up. It looked like the rate of rotation for one of the qubits was speeding up, which I’d never seen in four years of running these experiments,” said Tanttu, a measurement engineer at Diraq.

Tanttu had discovered a new way to manipulate the quantum state of a single qubit by using electric fields, rather than the magnetic fields used previously. Since the discovery was made in 2020, the engineers have been perfecting the technique.

“This is a new way to manipulate qubits, and it’s less bulky to build — you don’t need to fabricate cobalt micro-magnets or an antenna right next to the qubits to generate the control effect. It removes the requirement of placing extra structures around each gate, so there’s less clutter,” Gilbert said.

Controlling single electrons without disturbing others nearby is essential for quantum information processing in silicon. There are two established methods: ‘electron spin resonance’ (ESR) using an on-chip microwave antenna; and electric dipole spin resonance (EDSR), which uses an induced gradient magnetic field. The new technique is known as ‘intrinsic spin-orbit EDSR’.

“Normally, we design our microwave antennas to deliver purely magnetic fields. But this particular antenna design generated more of an electric field than we wanted — and that turned out to be lucky, because we discovered a new effect we can use to manipulate qubits. That’s serendipity for you,” Tanttu said.

Professor Andrew Dzurak, CEO and founder of Diraq, said this new mechanism adds to the trove of proprietary technology his team has developed over the past 20 years. “It builds on our work to make quantum computing in silicon a reality, based on essentially the same semiconductor component technology as existing computer chips, rather than relying on exotic materials. Since it is based on the same CMOS technology as today’s computer industry, our approach will make it easier and faster to scale up for commercial production and achieve our goal of fabricating billions of qubits on a single chip,” Dzurak said.

CMOS (or complementary metal-oxide-semiconductor) is the fabrication process that is used for making all sorts of integrated circuit components — including microprocessors, microcontrollers, memory chips and other digital logic circuits, as well as analog circuits such as image sensors and data converters. Building a quantum computer is thought to be a difficult and ambitious challenge with the potential to deliver new tools for tackling otherwise impossible calculations, such as the design of complex drugs and advanced materials, or the rapid search of massive, unsorted databases.

“We often think of landing on the Moon as humanity’s greatest technological marvel. But the truth is, today’s CMOS chips — with billions of operating devices integrated together to work like a symphony, and which you carry in your pocket — that’s an astounding technical achievement, and one that’s revolutionised modern life. Quantum computing will be equally astonishing,” Dzurak said.

Top image caption: How multiple qubits might be controlled using the new ‘intrinsic spin-orbit EDSR’ process. Image credit: Tony Melov.