Australian engineers have found a new way to precisely control individual electrons nestled in quantum dots that control logic gates.

This new mechanism is less bulky and requires fewer parts, which could be essential to realize large-scale silicon quantum computers.

The accidental discovery made by engineers at the start of quantum computing dirakthat rolled out UNSW Sydneyis described in the magazine Nature Nanotechnology.

Lead author Doctor Will Gilbert, a quantum processor engineer at Diraq, said that 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. a quantum bit (or qubit) exists in both states simultaneously, a state known as a “superposition.” This allows for a large number of computational strategies – some exponentially faster, some operating simultaneously – that are beyond classical computers

“This was a completely new effect that we had never seen before and that we didn’t fully understand at first,” he said.

“But it quickly became clear that this was a powerful new way to control spins in a quantum dot. And that was super exciting.”

Swap magnetic fields for electric ones

While experimenting with various geometric combinations of devices only one billionth of a meter in size controlling quantum dots, along with various types of tiny magnets and antennas controlling their operation, Dr. Tuomo Tantu stumbled upon a strange effect.

“I was trying to operate a gate with two qubits very precisely, going through a lot of different devices, slightly different geometries, different material stacks and different control techniques,” says Dr. Tanttu, a measurement engineer at Diraq.

“Then this strange spike popped up. It seemed that the rotational speed of one of the qubits was speeding up, which I had never seen in the four years that I had been conducting these experiments.”

What he had discovered, the engineers later realized, was a new way of manipulating the quantum state of a single qubit by using electric fields, instead of the magnetic fields they used before.

Since the discovery was made in 2020, the engineers have perfected the technique – which has become another tool in their arsenal to fulfill Diraq’s ambition to build billions of qubits on a single chip.

“This is a new way to manipulate qubits, and it’s less bulky to build — you don’t have to fabricate cobalt micromagnets or an antenna right next to the qubits to generate the control effect,” Gilbert said.

“It makes it unnecessary to place extra constructions around each gate. So there is less clutter.”

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

“Normally, we design our microwave antennas to provide purely magnetic fields,” says Dr Tanttu.

“But this particular antenna design generated more electric field than we wanted – which turned out to be fortunate, because we discovered a new effect that we can use to manipulate qubits. That’s serendipity for you.”

A success from one day to the next

In conversation with Startup Daily TV Professor Andrew DzurakCEO and founder of Diraq, and a professor of Quantum Engineering at UNSW, joked that Diraq was a “25-year overnight success.”

He led the team that won the first quantum logic gate in silicon in 2015 and the new mechanisms they developed are the result of more than two decades of research.

“It builds on our work to realize quantum computing in silicon, based on essentially the same semiconductor component technology as existing computer chips, rather than relying on exotic materials,” he said.

“Being based on the same CMOS technology as today’s computing industry, our approach will make it easier and faster to scale up for commercial production and reach our goal of fabricating billions of qubits on a single chip.”

CMOS (or Complementary Metal Oxide Semiconductor, pronounced “see-moss”) is the manufacturing process at the heart of modern computers. It is used to make all kinds 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.

“Today’s CMOS chips – with billions of control devices integrated together to work like a symphony, that you carry in your pocket – that is an astonishing technical achievement, and one that has revolutionized modern life,” said Prof. Dzurak.

“Quantum computing will be equally amazing.”

Click on the link below to hear more about Prof. Dzurak and Diraq’s breakthroughs.

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