Australian company claims to have found a way to control a billion quantum bits

In August 2021, a quantum team from the University of New South Wales (UNSW) in Australia touted the discovery of a new technique that will enable control of millions of spin quantum bits - the fundamental unit of information in a silicon quantum processor. Recently, the same team said [1] that a new method has been discovered to precisely control individual electrons in the quantum dots that run logic gates. More importantly, this new mechanism is less bulky and requires fewer components, which could prove to be the key to making large-scale silicon quantum computers a reality, even with a billion quantum bits.

A completely new way to control spin in quantum dots

Dr. Will Gilbert, a quantum processor engineer at Diraq, a spin-off company from UNSW, and the first author of this paper, said [2], "This was a completely new effect that we had never seen before, and we didn't quite understand it at first. But it quickly became clear that this is a powerful new way to control spin in quantum dots. It was super exciting."

Logic gates are the basic building blocks of all computing, allowing "bits" or binary numbers (0s and 1s) to work together to process information. However, a quantum bit exists in both states at the same time, a situation known as "superposition. Quantum superposition allows multiple computational strategies: exponential speed, simultaneous operation, which is unmatched by classical computers.

Quantum bits themselves consist of "quantum dots", tiny nanodevices that can capture one or a few electrons. Therefore, precise control of the electrons is essential for computation.

New approach: intrinsic spin-orbiting electric dipole spin resonance

While experimenting with different geometrical combinations of devices only a billionth of a meter in size that control quantum dots, as well as with the various types of miniature magnets and antennas that drive their operation, Dr. Tuomo Tanttu, a measurement engineer at Diraq, discovered a strange effect.

"I was trying to operate a two-qubit quantum gate really accurately, iterating through a lot of different devices, different geometries, different material stacks and different control techniques, and then this strange peak came up. It looked like one of the quantum bits was spinning faster, something I had never seen in the four years I had been running these experiments."

The engineers later realized that what he had discovered was a new way to manipulate the quantum states of individual quantum bits by using an electric field, rather than the magnetic field they had used before. Engineers have been refining the technique since its discovery in 2020; today, it has become another tool for realizing Diraq's ambition to build billions of quantum bits on a single chip.

The individual quantum bits held within the quantum dot flip in response to microwave signals.

Gilbert said, ""This is a new way to manipulate quantum bits and build them in a less bulky way. We don't need to build cobalt micromagnets or antennas next to the quantum bits to produce the control effect. It eliminates the requirement to place additional structures around each gate. Hence, more orderly.

Controlling individual electrons without disturbing other electrons in the vicinity is essential for quantum information processing in silicon. There are two established approaches: electron spin resonance (ESR) using on-chip microwave antennas; and electric dipole spin resonance (EDSR), which relies on an induced gradient magnetic field. The newly discovered technique is called "intrinsic spin-orbit EDSR.

"Normally, we design microwave antennas to provide pure magnetic fields," said Dr. Tanttu, "but this particular antenna design produced more electric fields than we wanted, which is fortunate because we discovered a new effect that can be used to manipulate quantum bits. ."

Pulsed electron spin-orbit spectroscopy (PESOS)

An aerial view of the Diraq lab in Sydney, Australia

Promising silicon quantum computing with a billion quantum bits

"This is a completely new mechanism that adds to the library of proprietary technologies we have developed over the past 20 years of research." Professor Andrew Dzurak, CEO and founder of Diraq and professor of quantum engineering at the University of New South Wales, who led the team that built the first quantum logic gate in silicon in 2015, said.

"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. Because it is based on the same CMOS technology as the computer industry today, our approach will make it easier and faster to scale to commercial production and achieve our goal of manufacturing billions of quantum bits on a single chip."

CMOS (complementary metal oxide semiconductor) is the core manufacturing process for modern computers. It is used to manufacture a variety 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.

An illustration of a single quantum bit, where electrons begin to wiggle inside a quantum dot as it begins to accelerate in response to a microwave signal.

Building quantum computers has been called the "space race of the 21st century" - a difficult and ambitious challenge that has the potential to provide revolutionary tools for solving otherwise impossible computations, such as the design of complex drugs and advanced materials, or the rapid search of large numbers of unclassified databases.

"We often think of the moon landing as man's greatest technological miracle," Dzurak said, "but the fact that today's CMOS chips have billions of operating devices integrated into them is an astounding technological achievement that has revolutionized modern life. Quantum computing will be equally astounding."

Although this company is very ambitious, there is a problem, whether their method can control millions or billions of quantum bits, it is not experimentally verifiable at this stage, because humans cannot yet achieve so many quantum bits.

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