A team of quantum physicists at UNSW Sydney has designed an atomic-scale quantum processor to simulate the behavior of a small organic molecule, solving a challenge posed about 60 years ago by theoretical physicist Richard Feynman.
The achievement, which took place two years earlier than expected, represents an important milestone in the race to build the world’s first quantum computer and demonstrates the team’s ability to control the quantum states of silicon electrons and atoms in a exquisite level that had not been achieved before.
In an article published today in the journal Nature, researchers described how they were able to mimic the structure and energy states of the organic compound polyacetylene, a repeated chain of carbon and hydrogen atoms distinguished by alternating single and double carbon bonds.
Leading Australian researcher of the year, Scientia Professor Michelle Simmons, said the Silicon Quantum Computing team, one of UNSW’s most exciting start-ups, built a quantum integrated circuit consisting of ‘a chain of 10 quantum dots to simulate the precise location of atoms. in the polyacetylene chain.
Artist print of the interior of the quantum integrated circuit modeling the carbon chain. The simulated carbon atoms are in red, while blue represents the electrons exchanged between them. Image: SQC
“Going back to the 1950s, Richard Feynman said you can’t understand how nature works unless you can build matter on the same length scale,” Professor Simmons said.
“And that’s what we’re doing. We’re literally building it from the bottom up, where we’re mimicking the polyacetylene molecule by putting silicon atoms at the exact distances that single and double carbon-carbon bonds represent.”
Chain reaction
The research was based on measuring the electric current using a deliberately designed 10-point quantum dot replicate of the polyacetylene molecule as each new electron passed from the source output of the device to the drain, the other end of the circuit.
To be doubly safe, they simulated two different threads from the polymer chains.
In the first device they cut a piece of the chain to leave double links at the end giving 10 peaks to the current. In the second device they cut a different fragment of the chain to leave simple links at the end only giving rise to two peaks of current. The current flowing through each chain was therefore very different due to the different bond lengths of the atoms at the end of the chain.
The measurements not only match the theoretical predictions, but they match perfectly.
“We should have some sort of business result from our technology in five years.” – Michelle Simmons
“What it’s showing is that you can literally mimic what’s really going on in the real molecule. And that’s why it’s exciting because the signatures of the two strings are so different,” said Professor Simmons.
“Most other quantum computing architectures out there do not have the ability to design atoms with subnanometric precision or allow atoms to settle so close together.
“And that means we can now begin to understand increasingly complicated molecules based on putting atoms in place as if they were mimicking the real physical system.”
Stand on the edge
According to Professor Simmons, it was no coincidence that a 10-atom carbon chain was chosen because it is within the size limit of what a classical computer is capable of calculating, with up to 1024 separate electron interactions in this system. . Increasing it to a 20-point chain would exponentially increase the number of possible interactions, making it difficult to resolve a classic computer.
“We’re close to the limit of what classic computers can do, so it’s like stepping from the edge into the unknown,” he says.
“And that’s exciting, now we can make bigger devices that go beyond what a classic computer can model. So we can look at molecules that haven’t been simulated before. We’ll be able to understand the world differently, addressing fundamental issues that we have never been able to resolve before ”.
One of the questions Professor Simmons alluded to is about understanding and mimicking photosynthesis: how plants use light to create chemical energy for growth. Or understand how to optimize the design of catalysts used for fertilizers, a currently high energy and high cost process.
“So there are big implications for understanding fundamentally how nature works,” he said.
Future quantum computers
Much has been written about quantum computers in the last three decades with the billion dollar question always being “but when can we see one?”
Professor Simmons says that the development of quantum computers is on a trajectory comparable to how classical computers evolved: from a transistor in 1947 to an integrated circuit in 1958, and then small computer chips that were incorporated into commercial products. like calculators about five years later. .
“And now we’re replicating this roadmap for quantum computers,” says Professor Simmons.
The authors of the article Nature in the Silicon Quantum Computing Laboratory.
“We started with a single-atom transistor in 2012. And this latest result, made in 2021, is the equivalent of the atomic-scale quantum integrated circuit, two years earlier. If we do this with the evolution of classical computing, we are predicting that we should have some sort of commercial result for our technology within five years. “
One of the advantages of the UNSW / SQC team’s research is that the technology is scalable because it manages to use fewer circuit components to control the qubits, the basic bits of quantum information.
“In quantum systems, you need something that creates qubits, some kind of structure in the device that allows you to form the quantum state,” says Professor Simmons.
Read more: UNSW Quantum Scientists Offer World’s First Atomic-Scale Integrated Circuit
“In our system, the atoms themselves create the qubits, and require fewer elements in the circuits. We only needed six metal gates to control the electrons in our 10-point system, which means we have fewer gates than the active components of the device. “While most quantum computing architectures require almost twice as much or more of control systems to move electrons into the qubit architecture.”
The need for fewer tightly packaged components minimizes the amount of any interference with quantum states, allowing devices to expand to make quantum systems more complex and powerful.
“So this low door physical density is also very exciting for us, because it shows that we have this clean and nice system that we can handle, maintaining consistency over long distances with minimal overload on the doors. That’s why it’s valuable for to scalable quantum computing “.
Looking to the future, Professor Simmons and her colleagues will explore larger compounds that may have been theoretically predicted but never before simulated and fully understood, such as high-temperature superconductors.