UNSW Sydney quantum engineers have overcome a significant hurdle that prevented quantum computers from becoming a reality. They discovered a new technique that would allow them to control millions of spin qubits, the basic units of information within a silicon quantum processor.
Scientists and quantum computer engineers have used a proof of concept model of quantum processors to demonstrate the control of a few qubits.
Their latest research was published in Science Advances today. The missing piece in quantum computer architecture should allow for control over the millions of qubits required to perform complex calculations.
Jarryd Pla is a UNSW’s School of Electrical Engineering and Telecommunications professor. His research team was trying to solve the problem that has stumped quantum computer scientists over decades: How to control not just a few but millions of qubits without taking up valuable space, using more electricity, and generating heat.
Dr. Pla states, “Up to this point, controlling electron spun qubits depended on us delivering microwave magnet fields by putting current through a wire just besides the qubit.”
“This presents real challenges if you want to scale up to the millions upon millions of qubits that quantum computers will need to solve significant global problems such as the design and manufacture of vaccines.
First, magnetic fields lose their effectiveness with distance. Therefore, we can only control qubits that are closest to the wire. As we add more qubits to the wire, we must increase the number of cables. This would eat up lots of chip real estate.
Because the chip is required to operate below -270degC, Dr. Pla states that adding more wires to the chip would cause too much heat, compromising the reliability of the qubits.
Dr. Pla states, “So we return to only being in a position to control a handful of qubits using this wiring technique.”
Moment of lightbulb
A complete redesign of the structure of the silicon chips solved this problem.
Instead of having thousands of control wires attached to a tiny silicon chip with millions of qubits, the team explored the possibility of creating a magnetic field above the flake to manipulate all qubits simultaneously.
In the 1990s, quantum computing scientists first proposed controlling all qubits simultaneously. However, until now, people have yet to find a practical way.
“We first removed the wire near the qubits, and then we came up with a new way to deliver microwave frequency magnetic control fields throughout the entire system. In principle, control fields could be delivered to as many as four million qubits,” Dr. Pla says.
Dr. Pla and his team created a new component above the silicon chip, a crystal prism known as a dielectric resonance. The resonator focuses microwave waves down to a smaller size when directed into it.
“The dielectric resonance shrinks the wavelength below one millimeter. This allows us to convert microwave power into a magnetic field that controls all the qubit spins.
There are two significant innovations. First, we can use a little power to generate a strong driving force for the qubits. This is crucial because it means that we produce little heat. The second is that the field is uniform across the chip so that all qubits have the same level of control.
Quantum team up
While Dr. Pla and his colleagues had created the prototype resonator technology but didn’t have silicon qubits on which to test it, He spoke to Andrew Dzurak, his UNSW engineering colleague, who had demonstrated over the past decade the most precise quantum logic using the same silicon manufacturing techniques that made conventional computer chips.
Prof. Dzurak said, “I was blown away when Jarryd approached me with his new concept.” He also says, “we immediately got to work to integrate it with the qubit chip that my team has created.”
“We sent two of our top Ph.D. students, Ensar Vasoglu from my team and James Slack-Smith from Jarryd’s to the project.
“We were thrilled when the experiment was successful. The problem of controlling millions of qubits was something that I had been worried about for some time. It was a significant roadblock in building a quantum computer.
Quantum computers that use thousands of qubits to solve commercial problems may be within a decade, something we had only imagined in the 1980s because they can model complex systems and provide new power to solve global issues and develop new technologies.
Quantum computing technology can improve lives in many areas, including climate change, vaccine design, code decryption, and artificial intelligence.
Looking ahead
The team will use the new technology next to make near-term silicon quantum processors more efficient.
“Removing on-chip control wire opens space for additional qubits and all the electronics necessary to build a quantum processor. Professor Dzurak says it makes it much easier to move on to producing devices with tens of thousands of qubits.
Dr. Pla says that while there are still engineering issues to solve before processors with one million qubits can become possible, he is excited to announce that he now has a way to manage them.
