Wednesday, February 8, 2023

A new way of precisely controlling single electrons nestled in quantum dots

Discovery of previously unknown effects makes compact, ultra-fast control of spin qubits possible.

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Once called a ‘classically non-describable two-valuedness’ by Pauli, the electron spin forms a qubit that is naturally robust to electric fluctuations. Paradoxically, a common control strategy is the integration of micromagnets to enhance the coupling between spins and electric fields, which, in turn, hampers noise immunity and adds architectural complexity.

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

This is the first time that scientists are seeing this effect. At first, scientists were clueless, but later they found that this was a powerful new way of controlling spins in a quantum dot. And that was super exciting.

Dr. Tuomo Tanttu from UNSW Engineering discovered a peculiar phenomenon when testing with various geometrical configurations of devices just billionths of a meter in size that govern quantum dots and numerous sorts of tiny magnets and antennas that drive their activities.

Dr. Tanttu said, “I was trying to accurately operate a two-qubit gate, iterating through a lot of different devices, slightly different geometries, different materials stacks, and different control techniques. Then this strange peak popped up. It looked like the rotation rate for one of the qubits was speeding up, which I’d never seen in four years of running these experiments.”

The engineers later realized that what he had uncovered was a novel method for controlling the quantum state of a single qubit by using electric fields instead of the magnetic fields they had previously used. The engineers have been honing the technology since its discovery in 2020. It has now become another weapon in their toolbox for realizing Diraq’s goal of assembling billions of qubits on a single chip.

Lead author Dr. Will Gilbert, a quantum processor engineer at Diraq, a UNSW spin-off company based at its Kensington campus, said, “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. It removes the requirement of placing extra structures around each gate. So, there’s less clutter.”

Two established methods for controlling single electrons without disturbing 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.’

Dr. Tanttu said, “Normally, we design our microwave antennas to deliver purely magnetic fields. But this particular antenna design generated more of an electric field than we wanted – but that turned out to be lucky because we discovered a new effect we can use to manipulate qubits. That’s serendipity for you.”

Professor Andrew Dzurak, Scientia Professor in Quantum Engineering at UNSW, said“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.”

“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.”

“Since it’s 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.”

Journal Reference:

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