Engineers make breakthrough in quantum computer design

Quantum engineers at UNSW Sydney have removed a major obstacle that stood in the way of quantum computers becoming a reality. They have discovered a new technique that they say will be able to control millions of spin qubits, the basic units of information in a silicon quantum processor.

Until now, quantum computer scientists and engineers had worked with a proof-of-concept model of quantum processors demonstrating the control of just a handful of qubits.

But with his latest research, published today in Science Advances, the team has found what it considers "the missing piece" in the architecture of quantum computers, which should allow control of the millions of qubits required to perform extraordinarily complex calculations.

Dr. rryd Pla, a professor at UNSW's School of Electrical Engineering and Telecommunications, says his research team wanted to solve the problem that had puzzled quantum computer scientists for decades: how to control not just a few qubits. If not millions, without taking up valuable space with more wiring, consuming more electricity and generating more heat.

"Until now, the control of the qubitsThe spin of electrons depended on us supplying microwave magnetic fields by putting a current through a wire right next to the qubit, "says Dr. Pla.

" This raises some real problems if we want to scale up to millions of qubits. that you will need a quantum computer to solve problems of global importance, such as the design of new vaccines.

"First, magnetic fields decrease very rapidly with distance, so we can only control the qubits closest to the wire. That means we would have to add more and more cables as we put in more and more qubits, which would take up a lot of space on the chip."

And since the chip must operate at very low temperatures, below -270 ° C, Dr. Pla says that inserting more wires would generate too much heat in the chip, which would interfere with the reliability of the qubits.

"So we are again able to control only a few qubits with this wire technique," says Dr. Pla.

The moment of the light bulb

The solution to this problem involved a complete reimagining of the silicon chip structure.

Rather than having thousands of control wires on the same miniature-sized silicon chip that also has to contain millions of qubits, the team studied the possibility of generating a magnetic field from above the chip that could manipulate all qubits simultaneously. .

This idea of ​​controlling all qubits simultaneously was first brought up by quantum computer scientists in the 1990s, but until now no one had found a practical way to do it.

"We first removed the wire next to the qubits and then came up with a novel way of supplying microwave frequency magnetic control fields throughout the system. So, in principle, we could supply control fields at up to four million qubits," he says. Dr. Pla.

Dr. Pla and his team introduced a new component directly on top of the silicon chip: a crystal prism called a dielectric resonator. When the microwaves are directed at the resonator, the resonator focuses the wavelength of the microwaves to a much smaller size.

The dielectric resonator reduces the wavelength to below a millimeter, so we now have a very efficient conversion of microwave energy into the magnetic field that controls the spins of all qubits.

"" There are two key innovations. The first is that we don't have to put in a lot of energy to get a strong conduction field for the qubits, which basically means that we don't generate a lot of heat. The second is that the field is very uniform across the chip, so millions of qubits experience the same level of control. "

Quantum equipment

Although Dr. Pla and his team had developed the prototype for the resonator technology, they did not have the silicon qubits to test them. So he spoke to his UNSW engineering colleague, Scientia professor Andrew Dzurak, whose team had demonstrated in the last decade the first and most accurate quantum logic using the same silicon fabrication technology used to make conventional computer chips.

"I was completely amazed when Jarryd came to see me with his new idea," says Professor Dzurak, "and we immediately got 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.

"We were very happy when the experiment was successful. This problem of how to control millions of qubits had worried me for a long time, as it was a major obstacle to building a large-scale quantum computer."

Quantum computers, once only dreamed of in the 1980s, using thousands of qubits to solve commercially important problems, may now be less than a decade away. Beyond that, they are expected to bring new firepower to solving global challenges and developing new technologies thanks to their ability to model extraordinarily complex systems.

Climate change, drug and vaccine design, code breaking, and artificial intelligence can all benefit from quantum computing technology.

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