Breakthrough could be key to large-scale quantum computers

Quantum computers promise to revolutionize computing, but the technology is not without its problems.

Research from the University of New South Wales (UNSW) in Australia may bring us closer to solving one of the main problems: how to control individual qubits in a series of qubits.

Most quantum computers today operate on a very small number of quantum bits (or qubits). Late last year, IBM announced that they have a quantum computer, the IBM Osprey, with 433 qubits. That’s more than three times the size of the previous largest quantum processor, the IBM Eagle, with 127 qubits.

As significant as these developments are, to be honest, running a quantum computer with more than three or four qubits is a nightmare. The same quantum effects that promise to make quantum computers so powerful also make it incredibly difficult.

Quantum effects mean that controlling individual qubits without disturbing other qubits is extremely difficult. Most quantum computer architectures are bulky and complex as engineers try to suppress or compensate for quantum disturbances.

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UNSW research involving quantum computing start-up Diraq has revealed a new way to precisely control individual electrons in a series of qubits (sometimes called “quantum dots” in some architectures).The discovery was published in Nature Nanotechnology.

“This is a completely new effect that we hadn’t seen before and we didn’t understand very well at first,” said lead author Dr. Will Gilbert, an engineer at Diraq. “But it quickly became clear that this was a powerful new way to control the spin of quantum dots. That was really exciting.”

The team encountered a strange effect while experimenting with the geometric arrangement of the nanoscale devices.

“I was trying to operate a two-qubit gate really accurately, iterating through many different devices, slightly different geometries, different material stacks and different control techniques,” explained co-author and Diraq engineer Dr. Tuomo Tanttu. “Then this weird mountain suddenly appeared. It looked like one of the qubits was spinning faster, which I hadn’t seen in the four years I’ve been doing these experiments.”

What Tanttu and team discovered completely by accident was a new way to manipulate the quantum state of a single qubit using an electric field. Previously, they had been trying to use magnetic fields for single-qubit control.

“Typically, we design microwave antennas to provide a pure magnetic field,” comments Dr. Tanttu. “But this particular antenna design produced more electric field than we wanted – but it turned out to be lucky, because we found a new effect that can be used to manipulate qubits. This will come as a surprise to you joy.”

After making the discovery in 2020, Diraq engineers have been refining their technique, which they hope will eventually allow them to build a single chip with billions of qubits.

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“It’s a new way to manipulate qubits, and it’s much smaller to build—you don’t need cobalt micromagnets or antennas next to the qubits to have the control effect,” Gilbert added. “It eliminates the requirement to put extra structure around each door. So, less clutter.”

“This is a gem of a new mechanism that only adds to the treasure trove of know-how we have developed over the past 20 years of research,” said Diraq founder and CEO Professor Andrew Dzurak. “It builds on our work in making 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 the computer industry today, our approach will Making it easier and faster to scale up commercial production and achieve our goal of making billions of qubits on a single chip.”

“We often think of the moon landing as humanity’s greatest technological marvel,” Dzurak said. “But the fact is that today’s CMOS chip — integrating billions of operating devices together, working like a symphony, and you can take it with you — is an amazing technological achievement that has revolutionized modern life. Quantum computing will be equally amazing.”

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