Quantum computers promise to revolutionise computing, but the technology is not without its problems.
Research coming out of Australia’s University of New South Wales (UNSW) might take us a step closer to tackling one of the major problems: how to control single quantum bits in a series of them.
Most quantum computers today run on a very small number of quantum bits (or qubits). Late last year, IBM announced that they had a quantum computer, IBM Osprey, with 433 qubits. This more than tripled the previous largest quantum processor which had 127 qubits, IBM Eagle.
As significant as these developments are, if we’re speaking honestly, a quantum computer with more than three or four qubits is a nightmare to run. The very same quantum effects which promise to make quantum computers so powerful, are also making it very difficult to build.
Quantum effects mean controlling individual qubits without interfering with the others is extremely difficult. Most quantum computer architectures are bulky and complicated in engineers’ attempts to either dampen the quantum interference, or compensate for it.
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Research at UNSW involving the quantum computing start-up Diraq reveals a new way of precisely controlling single electrons within a series of qubits (sometimes referred to as “quantum dots” in certain architectures). The discovery is published in Nature Nanotechnology.
“This was a completely new effect we’d never seen before, which we didn’t quite understand at first,” says lead author Dr Will Gilbert, an engineer at Diraq. “But it quickly became clear that this was a powerful new way of controlling spins in a quantum dot. And that was super exciting.”
The team came across a strange effect while experimenting with the geometrical arrangement of devices nanometres in size.
“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,” explains co-author and Diraq engineer Dr Tuomo Tanttu. “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.”
What Tanttu and the team had discovered completely by accident was a new way of manipulating the quantum state of a single qubit using electrical fields. Previously, they had been attempting single qubit control using magnetic fields.
“Normally, we design our microwave antennas to deliver purely magnetic fields,” Dr Tanttu remarks. “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.”
Having made the discovery in 2020, Diraq engineers have since been refining their technique which they hope will eventually lead to the building of single chips with billions of qubits on them.
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“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.”
“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,” says Diraq founder and CEO Professor 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. 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.”
“We often think of landing on the Moon as humanity’s greatest technological marvel,” says Dzurak. “But the truth is, today’s CMOS chips – with billions of operating devices integrated together to work like a symphony, and that you carry in your pocket – that’s an astounding technical achievement, and one that’s revolutionised modern life. Quantum computing will be equally astonishing.”