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Quantum engineers from UNSW Sydney have now solved a problem that has baffled scientists for decades: How to reliably control millions of qubits in a silicon quantum computer chip without wasting valuable space with extra wiring.

This issue had been a significant roadblock to the development of a full-scale quantum computer, but it has now been overcome thanks to the engineers who developed a new technique capable of controlling millions of spin qubits simultaneously.

“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 leader Dr. Jarryd Pla said, a faculty member in UNSW’s School of Electrical Engineering and Telecommunications, in a press release by the university.

“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,” Pla explained.

Furthermore, because the chip must operate at freezing cold temperatures, adding more wires would generate far too much heat in the chip, compromising the qubits’ reliability.

Finding “the missing jigsaw piece”

The team claims to have found “the missing jigsaw piece” in the quantum computer architecture in a paper published in Science Advances, which should enable them to manage the millions of qubits needed for extremely complicated computations.

Their solution is based on a complete rethinking of silicon chip architecture: Rather than putting thousands of control wires on a tiny silicon device that also has millions of qubits, the researchers investigated the possibility of using a magnetic field generated from above the chip to operate all of the qubits at the same time.

The concept of controlling all qubits at the same time was originally proposed by quantum computing experts in the 1990s, according to the press release, but no one had found a viable method to do so until today.

The solution: A dielectric resonator

The engineers found the solution to this decades-old problem by adding a new component just above the silicon chip — a crystal prism known as a dielectric resonator. The neat trick here is that, when microwaves are directed into the resonator, the resonator focuses the wavelength of the microwaves down to a much smaller size, so the engineers achieve “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”, Pla explained. “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.”

When the resonator technology was tested, the tests were successful, and today, thanks to this development, ​quantum computers with thousands of qubits to tackle commercially significant issues may be less than a decade away.

“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.

The team’s next goal is to utilize this new technique to make designing near-term silicon quantum processors easier. Quantum computing technology has the potential to help climate change, drug and vaccine development, code decryption, and artificial intelligence.

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