UCLA-led team develops new approach to building quantum computers

Stephan Sullivan

An artist’s representation shows the researchers’ quantum functional groups (bright colored spheres) linked to larger molecules.

Main takeaways:

• Stronger, faster. Quantum computers promise far greater speed and processing power than today’s most advanced supercomputers
• Quantum quantum. As these next-generation computers relied on the interaction of fragile atomic and subatomic particles, increasing their processing power proved to be a challenge.
• A chemical solution. Researchers have created a new molecule that has the potential to protect quantum interactions on a larger scale without the need for traditional electrical engineering tools and machines.

Quantum computing, although still in its infancy, has the potential to dramatically increase processing power by exploiting the odd behavior of particles at the smallest scales. Several research groups have reported performing calculations that would take thousands of years for traditional supercomputers. In the long term, quantum computers can provide unbreakable encryption and simulation of nature beyond current capabilities.

An interdisciplinary research team led by UCLA including collaborators at Harvard University has now developed a fundamentally new strategy for building this computer. While the current state of the art uses circuits, semiconductors, and other electrical engineering tools, the team has produced a game plan based on the ability of chemists to design the building blocks of atoms that control the structural properties of larger molecules when they are placed. together.

The findings, published last week in Nature Chemistry, could eventually lead to a leap in the power of quantum processing.

“The idea is, instead of building a quantum computer, let chemistry build it for us,” said Eric Hudson, UCLA’s President David S. Saxon Professor of Physics and co-author of the study. “We’re all still learning the rules for this type of quantum technology, so this work is very sci-fi right now.”

The basic unit of information in traditional computing is the bit, each of which is limited to one of only two values. In contrast, a group of quantum bits — or qubits — can have a much wider range of values, exponentially increasing the processing power of a computer. Over 1,000 normal bits are required to represent only 10 qubits, whereas 20 qubits require more than 1 million bits.

That characteristic, at the heart of the transformational potential of quantum computing, hinges on the opposite rule that applies when atoms interact. For example, when two particles interact, they can become linked, or become entangled, so that measuring the property of one determines the property of the other. Involving qubits is a quantum computing requirement.

However, this attachment is fragile. When qubits encounter subtle variations in their environment, they lose their “quantity,” which is needed to implement quantum algorithms. This limits the most powerful quantum computers to less than 100 qubits, and keeping these qubits in a quantum state requires large machines.

To practically apply quantum computing, engineers will have to increase that processing power. Hudson and his colleagues believe they have made the first steps with this research, in which theory guides the team to create molecules that protect quantum behavior.

The scientists developed a small molecule that includes calcium and oxygen atoms and acts as a qubit. This calcium-oxygen structure forms what chemists call a functional group, meaning that it can be linked to almost any other molecule while also giving that molecule its own properties.

The team showed that their functional group retains the desired structure even when attached to much larger molecules. Their qubits are also resistant to laser cooling, a key requirement for quantum computing.

“If we can bind quantum functional groups to surfaces or long molecules, we may be able to control more qubits,” Hudson said. “It should be cheaper to upgrade, because atoms are one of the cheapest things in the universe. You can make as many as you want. ”

In addition to their potential for next-generation computing, quantum functional groups can benefit fundamental discoveries in chemistry and the life sciences, for example by helping scientists uncover more about the structure and function of various molecules and chemicals in the human body. .

“Qubits can also be very sensitive tools for measurement,” said study co-author Justin Caram, UCLA assistant professor of chemistry and biochemistry. “If we can protect them so that they can survive in complex environments such as biological systems, we will be armed with so much new information about our world.”

Hudson said that the development of a chemistry-based quantum computer could realistically take decades and was unlikely to succeed. Future steps include tethering qubits to larger molecules, coaxing tethered qubits to interact as processors without unwanted signaling, and entangling them so that they function as a system.

The project was seeded by a Department of Energy grant that gave physicists and chemists the opportunity to bypass discipline-specific jargon and speak a common scientific language. Caram also praised UCLA’s easy collaboration atmosphere.

“This is one of the most intellectually satisfying projects I’ve ever worked on,” he said. “Eric and I first met at lunch at the Faculty Center. It’s born out of fun conversations and being open to talking to new people.”

UCLA postdoctoral researcher Guo-Zhu Zhu is the study’s first author. Other UCLA co-authors are doctoral students Claire Dickerson and Guanming Lao and faculty members Anastassia Alexandrova and Wesley Campbell.

The study was also supported by the National Science Foundation, Army Research Office and Air Force Office of Scientific Research.

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