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The Serendipitous Discovery: ‘A Powerful New Way Of Controlling Spins In A Quantum Dot’

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Scientists have discovered a new, more efficient method for manipulating qubits. This new approach eliminates the need for bulky components such as cobalt micro-magnets or antennas placed near the qubits, making the building process less complex and more streamlined.

Engineers from Australia have found a new way to control single electrons in quantum dots, which are used to run logic gates.

Additionally, the new process is less cumbersome and calls for fewer components, both of which may prove to be vital in the development of large-scale silicon quantum computers in the future.

The paper describing the work was published today in the journal Nature Nanotechnology.

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

All processing relies on logic gates, which connect groups of “bits,” or binary digits (0s and 1s), to perform logical operations.

A quantum bit (also known as a qubit) may, 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.

Quantum dots, which are small nanodevices with the ability to trap one or more electrons, are the building blocks of qubits. For computing to take place, the electrons must be precisely controlled.

Using electric fields as opposed to magnetic fields

Dr. Tuomo Tanttu found a strange effect while he was playing around with different geometrical combinations of devices that are billionths of a meter in size and control quantum dots, as well as different types of magnets and antennas that make them work.

“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,” adds Dr. Tanttu from Diraq. “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.”

The engineers eventually realized that what he had found 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 method since its discovery in 2020; it is now one of several tools at their disposal as they work to realize Diraq’s goal of creating 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 need to fabricate cobalt micro-magnets or an antenna right next to the qubits to generate the control effect,” adds Gilbert. “It removes the requirement of placing extra structures around each gate. So, there’s less clutter.”

For quantum information processing in silicon, controlling a single electron without affecting surrounding neighbors is crucial. There are two well-known techniques: electric dipole spin resonance (EDSR), which makes use of an induced gradient magnetic field, and “electron spin resonance” (ESR), which uses an on-chip microwave antenna. “intrinsic spin-orbit EDSR” is the name of the new technique.

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

Silicon quantum computing gets closer thanks to discovery

“This is a gem of new mechanism, which just adds to the trove of proprietary technology we’ve developed over the past 20 years of research,” points out lead author Prof Andrew Dzurak.

“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,” he adds. “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.”

CMOS, or complementary metal-oxide-semiconductor, is the manufacturing technique at the core of contemporary computers. It is used to create a variety of integrated circuit parts, including as image sensors, data converters, and digital logic circuits like microprocessors, microcontrollers, memory chips, and other digital logic circuits.

The race to develop a practical quantum computer has been dubbed the “space race of the 21st century,” and it promises to be an extremely difficult and ambitious undertaking that could result in revolutionary tools for performing previously impossible calculations, such as the design of complex drugs and advanced materials or the rapid search of massive, unsorted databases.

The Moon landing is often regarded as humanity’s greatest technical achievement, according to Dzurak. But 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 astonishing technological feat that have completely changed contemporary living. It will be as astounding to use quantum computers.

Source: 10.5281/zenodo.7223114

Image Credit: Tony Melov

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