Once called a “classically indescribable divalent” by Pauli, the electron spin forms a qubit that is naturally robust to electrical fluctuations. Paradoxically, a common control strategy is to integrate micromagnets to improve coupling between spins and electric fields, which in turn hinders noise immunity and adds architectural complexity.

Engineers at UNSW Sydney have developed a new method for precisely regulating individual electrons tucked away in quantum dots that operate logic gates. The new technique is also less complex and requires fewer components, which could be crucial for realizing large-scale silicon quantum computers.

This is the first time scientists have seen this effect. At first, scientists had no idea, but they later discovered that this was a powerful new way to control spins in a quantum dot. And that was super exciting.

Dr. Tuomo Tanttu of UNSW Engineering discovered a peculiar phenomenon when testing different geometric configurations of devices only one billionth of a meter in size that control quantum dots and countless types of tiny magnets and antennas that direct their activities.

Doctor Tanttu said: “I was trying to precisely control a gate with two qubits, going through many different devices, slightly different geometries, different material stacks, and different control techniques. 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 of conducting these experiments.”

The engineers later realized that what he had discovered was a new method of controlling the quantum state of a single qubit by using electric fields instead of the magnetic fields they had used before. The engineers have honed the technology since its discovery in 2020. It has now become another weapon in their toolbox to realize Diraq’s goal of merging billions of qubits on a single chip.

Lead author Dr. Will Gilbert, a quantum processor engineer at Diraq, a UNSW spin-off company based on the Kensington campus, said: “This is a new way of manipulating 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. It removes the requirement to put additional structures around each gate, so there is less clutter.”

Two established methods for controlling single electrons without interfering with others are 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.”

Doctor Tanttu said: “Normally, we design our microwave antennas to provide pure magnetic fields. But this particular antenna design generated more electric field than we wanted – which turned out to be lucky, because we discovered a new effect that we can use to manipulate qubits. That’s serendipity to you.”

Professor Andrew Dzurak, Scientia Professor of Quantum Engineering at UNSW, said: “This is a gem of a new mechanism, which only adds to the wealth of proprietary technology we’ve developed over the past 20 years 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.”

“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.”

Magazine reference:

  1. Gilbert, W., Tanttu, T., Lim, WH et al. On-demand electrical control of spin qubits. Wet. Nanotechnology. (2023). DOI: 10.1038/s41565-022-01280-4