Quantum engineers from UNSW Sydney have made a critical breakthrough in the development of quantum computing technology, solving a problem that has long frustrated scientists and until now represented a major roadblock to the development of the next generation of computers. The problem in question involves spin qubits, which are the basic units of information in a silicon quantum processor. But first, what is a silicon quantum processor?
In classic computing, information is represented with electric charges in silicon: in quantum computing, information will be conveyed through ‘spin’, the property of an electron or atom that gives it magnetism.
So, a silicon quantum processor is the core of a quantum computer, and a ‘spin qubit’ is a unit of information conveyed via the spin of the electrons therein. Confused yet?
The breakthrough the team have made concerns these spin qubits, which traditionally have been labour-intensive to control.
“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,” says Jarryd Pla, lead researcher.
“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.”
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“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.”
And since the chip must operate at freezing cold temperatures, below -273°C, Pla says introducing more wires would generate way too much heat in the chip, interfering with the reliability of the qubits.
“So, we come back to only being able to control a few qubits with this wire technique.”
To circumvent this problem, the team realised they’d need to reimagine the structure of the silicon chip. Instead of involving wires, they suggested generating a magnetic field from above the chip that could manipulate all of the qubits at once.
The prospect of controlling all the qubits at once using a magnetic field had first been posited in the 1990s, but until now no one had developed a practical way to do 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,” says Pla.
Pla and the team then introduced a new component directly above the silicon chip – a crystal prism called a dielectric resonator. When microwaves are directed into the resonator, it focuses the wavelength of the microwaves down to a much 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.
“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.”
The team worked with UNSW professor Andrew Dzurak, whose team have over the past decade demonstrated the first and the 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 says, “and we immediately got down to work to see how we could integrate it with the qubit chips that my team has developed.
“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.”
The team hope to use this new innovation to simplify the design of quantum computers.
“Removing the on-chip control wire frees up space for additional qubits and all of the other electronics required to build a quantum processor. It makes the task of going to the next step of producing devices with some tens of qubits much simpler,” says Dzurak.
“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,” says Pla.
This is not the first time quantum engineers at UNSW Sydney have broken ground in the journey towards a quantum future: in April 2020, a team led by Dzurak published a proof-of-concept quantum processor unit that could allow quantum computers to work at 1.5 kelvin – 15 times warmer than quantum processors could previously work at (typically, quantum computers need to be only fractions of a degree above absolute zero in order to function), reducing the need for refrigerating equipment that costs millions of dollars.
Quantum computers, when they become a practical, scalable reality, will allow for extraordinarily fast problem-solving, processing troves of data that would take a typical computer far longer. Potential applications could range from creating innovative new medical treatments to pricing financial instruments.
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