billion-qubit quantum processors are getting closer thanks to a new spin control technique

 By UNSW sydney

Jan 12, 2023

How the novel "intrinsic spin-orbit EDSR" technique may be used to control multiple qubits. Thanks to Tony Melov

                            A novel technique for precisely regulating individual electrons in quantum dots, which operate logic gates, has been developed by Australian experts. Additionally, the new technique is more compact and requires fewer components, which may be crucial for realising large-scale silicon quantum computers.

The accidental finding, achieved by engineers from UNSW Sydney and the quantum computing start-up Diraq, is described in the journal Nature Nanotechnology.

Lead author Dr. Will Gilbert, a quantum processor engineer at Diraq, a UNSW spin-off business with headquarters in Sydney, stated, "This was a completely new phenomenon we'd never seen before, which we didn't quite comprehend at first." "But it soon became obvious that this was a potent new technique for manipulating spins in a quantum dot. And that was really thrilling."

The fundamental unit of computing, the logic gate, is how binary digits (0 and 1), or "bits," interact to process data. A quantum bit (also known as a qubit) can, however, exist simultaneously in both of these states; this is referred to as a "superposition." This opens up a wide range of calculation techniques that go beyond those of conventional computers, some of which are exponentially quicker and operate concurrently. The "quantum dots" that make up qubits are microscopic nanodevices with the ability to trap one or more electrons. For computing to take place, the electrons must be precisely controlled.

Engineers at Diraq have developed a novel method for precisely regulating individual electrons embedded in quantum dots that function as logic gates, bringing the possibility of billion-qubit quantum processors closer to reality. Additionally, the new technique is more compact and requires fewer components, which may be crucial for realising large-scale silicon quantum computers. Source: Diraq

Using electric rather than magnetic fields

Dr. Tuomo Tanttu discovered an odd phenomenon when testing with various geometrical arrangements of devices just billionths of a metre in size that govern quantum dots, as well as various kinds of tiny magnets and antennae that drive their activities.

Dr. Tanttu, a measurement engineer at Diraq, remembers, "I was attempting to very properly run a two-qubit gate, iterating through a number of various devices, somewhat different geometries, different materials stacks, and different control strategies." "Then a peculiar peak appeared. In four years of conducting these tests, I had never noticed the rate of rotation for one of the qubits accelerating."

The engineers eventually understood that what he had uncovered was a novel method for controlling the quantum state of a single qubit by employing electric fields as opposed to the magnetic fields they had previously used. The developers have been honing the technology since it was discovered in 2020, adding it to their toolbox as they work to realise Diraq's vision of creating billions of qubits on a single chip.

A single qubit contained within a quantum dot flips in response to a microwave transmission in this artist's conception. Thanks to Tony Melov

Creating cobalt micro-magnets or an antenna directly next to the qubits to provide the control effect is not necessary with this new method of manipulating qubits, according to Gilbert. "It does away with the need for additional buildings to be built around each gate. Therefore, there is less clutter."

For quantum information processing in silicon, controlling a single electron without affecting surrounding neighbours is crucial. Both "electron spin resonance" (ESR) and "electric dipole spin resonance" (EDSR), which both rely on an induced gradient magnetic field, are well-established techniques. The recently found method is called "intrinsic spin-orbit EDSR."

Dr. Tanttu stated, "Normally, we construct our microwave antennas to deliver exclusively magnetic fields. "However, the fact that this specific antenna design produced more of an electric field than we desired was fortunate since it allowed us to identify a brand-new phenomenon that we can utilise to control qubits. That's chance for you, then."

Bird's view of a Diraq laboratory in Sydney, Australia. Shaun Dougherty is to blame.

Discovery brings silicon quantum computing closer

"This is a gem of a new mechanism, which just adds to the trove of proprietary technology we've developed over the past 20 years of research," said Prof. Andrew Dzurak, CEO and Founder of Diraq and Professor in Quantum Engineering at UNSW. Prof. Dzurak was in charge of the group that constructed the first quantum logic gate in silicon in 2015.

The development "builds on our effort to make quantum computing on silicon a reality, based on basically the same semiconductor component technology as present computer chips," the scientist continued. Our method will make it simpler and quicker to scale up for commercial manufacturing and reach our objective of generating billions of qubits on a single chip since it is based on the same CMOS technology as today's computer industry.

Modern computers are made using a technique called complementary metal-oxide semiconductor, or CMOS (pronounced "see-moss"). All different types of integrated circuit parts, including microprocessors, microcontrollers, memory chips, and other digital logic circuits, as well as analogue circuits like image sensors and data converters, are made using it.

Illustration of a single qubit as the electron in the quantum dot starts to jiggle and rapidly accelerates in response to a microwave signal. Thanks to Tony Melov

The development of a quantum computer has been dubbed the "space race of the twenty-first century"; it is a challenging and ambitious undertaking that has the potential to produce ground-breaking tools for tackling calculations that would otherwise be insurmountable, such as the design of sophisticated drugs and advanced materials or the quick search of enormous, unsorted databases.

The Moon landing is frequently regarded as humanity's greatest technical achievement, according to Dzurak. "However, the reality is that today's CMOS chips, which you carry in your pocket and include billions of working devices combined together to function like a symphony, are an amazing technical feat that have completely changed modern living. It will be as amazing to see quantum computers."