In preparation for measurement, the spin qubit device is connected to a circuit board. Credit: Serwan Asaad
UNSW Sydney quantum engineers have eliminated a major hurdle that was holding back the realization of quantum computers. The new technique, which they claim will allow them to control millions of spin qubits-the fundamental units of information within a silicon quantum processor -- was discovered by UNSW Sydney's quantum engineers.
Scientists and quantum computer engineers have used a proof of concept model of quantum processors to demonstrate the control of a few qubits.
The team's latest research was published in Science Advances today. It reveals what they believe is the missing piece in quantum computer architecture. This should allow for control over the millions of qubits required to perform complex calculations.
Jarryd Pla is a UNSW faculty member. His research team sought to solve the problem that has stumped quantum computer scientists over decades. How to control not just millions but many qubits, without taking up valuable space and using more wiring which generates more electricity and heat.
Dr. Pla states that until now, the only way to control electron spin qubits was to deliver microwave magnetic fields through a wire directly beside it.
"This presents real challenges if you want to scale up to the millions upon millions of qubits that quantum computers will need to solve global significant 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 will need to increase the number of wires. This would eat up lots of chip real estate.
Because the chip is required to operate below -270C, Dr. Pla states that adding more wires would cause too much heat and compromise 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 wire technique."
Moment of lightbulb
This problem was solved by reimagining the structure of silicon chips.
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 chip that could simultaneously manipulate all qubits.
In the 1990s, quantum computing scientists first proposed the idea of controlling all qubits simultaneously. However, no one had yet found a practical way to do so.
"First, we took out the wire near the qubits. 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 wavelengths to a smaller size when microwaves are directed towards 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 major innovations. First, we don’t need to use a lot power to generate a strong driving force for the qubits. This is crucial because it means that we don’t produce much 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 first and most precise quantum logic using the same silicon manufacturing process used to create conventional computer chips.
"I was completely stunned when Jarryd came up to me with his idea," Prof. Dzurak said. "We immediately set to work to integrate it with the qubit chip that my team had developed."
"We sent Ensar Vahapoglu, a member of my team, along with 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 ten years, something we had only imagined in the 1980s. Because they can model complex systems, they will be able to provide new power to solve global problems and develop new technologies.
Quantum computing technology can be used to improve the lives of many areas, including climate change, vaccine design, code decryption, artificial intelligence, and drug and vaccine development.
The team will use the new technology next to make near-term silicon quantum processors more efficient.
"Removing on-chip control wire opens up 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.
Information: Science Advances, Single-electron Spin Resonance in a Nanoelectronic Device Using a Global Field. (2021). Information from Science Advances Single electron spin resonance in nanoelectronic devices using a global field (2021). DOI: 10.1126/sciadv.abg9158