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Advancing quantum computer design with new technology at UNSW

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Quantum engineers at the University of New South Wales (UNSW) have discovered a new technique capable of controlling millions of spin qubits – the basic units of information in a silicon quantum processor – that could lead to quantum computer development. 

Previously, quantum computer engineers and scientists worked with a proof-of-concept model of quantum processors by demonstrating the control of a handful of qubits. However, their latest research published in Science Advances has uncovered the missing piece in the quantum computer architecture to control the millions of qubits needed for complex calculations. 

UNSW’s School of Electrical Engineering and Telecommunications research team wanted to control millions of qubits without taking up valuable space with more wiring, using more electricity and generating more heat. 

“Up until this point, controlling electron spin qubits relied on us delivering microwave magnetic fields by putting a current through a wire right beside the qubit,” team faculty member Dr Jarryd Pla said. 

“This poses some real challenges if we want to scale up to the millions of qubits that a quantum computer will need to solve globally significant problems, such as the design of new vaccines. 

“First off, the magnetic fields drop off really quickly with distance, so we can only control those qubits closest to the wire. That means we would need to add more and more wires as we brought in more and more qubits, which would take up a lot of real estate on the chip.” 

The chip must operate below -270°C, so introducing more wires would generate too much heat in the chip and interfere with the reliability of the qubits. 

“So, we come back to only being able to control a few qubits with this wire technique,” Pla said. 

The technology 

The solution was generating a magnetic field from above the chip to manipulate all the qubits simultaneously. 

quantum computer
A spin qubit being connected to a circuit board, in preparation for measurement.

This idea was first proposed by quantum computing scientists in the 1990s, but nobody had worked out a practical way to achieve it. 

“First, we removed the wire next to the qubits and then came up with a novel way to deliver microwave-frequency magnetic control fields across the entire system. So in principle, we could deliver control fields to up to four million qubits,” Pla said. 

The team introduced crystal prism, called a dielectric resonator, directly above the silicon chip. When microwaves are directed into the resonator, it focuses the wavelength of the microwaves down to a smaller size. 

“The dielectric resonator shrinks the wavelength down below one millimetre, so we now have a very efficient conversion of microwave power into the magnetic field that controls the spins of all the qubits,” Pla said. 

“There are two key innovations here. The first is that we don’t have to put in a lot of power to get a strong driving field for the qubits, which crucially means we don’t generate much heat. The second is that the field is very uniform across the chip, so that millions of qubits all experience the same level of control.” 

Integrating it with qubits 

On developing the prototype resonator technology, the UNSW research team then spoke with UNSW Scientia Professor Andrew Dzurak. His team had demonstrated the first and most accurate quantum logic, using the same silicon manufacturing technology used to make conventional computer chips. 

“I was completely blown away when Jarryd came to me with his new idea,” Dzurak said. 

“We immediately got down to work to see how we could integrate it with the qubit chips that my team has developed. We put two of our best PhD students on the project, Ensar Vahapoglu from my team and James Slack-Smith from Jarryd’s. 

“We were overjoyed when the experiment proved successful. This problem of how to control millions of qubits had been worrying me for a long time, since it was a major roadblock to building a full-scale quantum computer.” 

Developing quantum computers 

If successful, quantum computers could help solve global and commercial problems and develop new technologies, through the ability to model complex systems. This includes climate change, drug and vaccine design, code decryption and artificial intelligence. 

The UNSW team plans to use their new technology to simplify a near-term silicon quantum processors’ design. 

“Removing the on-chip control wire frees up space for additional qubits and all of the other electronics required to build a quantum processor,” Dzurak said. 

“It makes the task of going to the next step of producing devices with some tens of qubits much simpler.” 

“While there are engineering challenges to resolve before processors with a million qubits can be made, we are excited by the fact that we now have a way to control them,” Pla said. 

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