Developing a new approach to building quantum computers

Quantum computing, while still in its infancy, has the potential to dramatically increase processing power by taking advantage of the strange behavior of particles at the smallest scales. Some research groups have already reported performing calculations that would take a traditional supercomputer thousands of years. In the long term, quantum computers could provide unbreakable encryption and simulations of nature beyond current capabilities.

An interdisciplinary research team led by UCLA, including collaborators from Harvard University, has now developed a fundamentally new strategy for building these computers. While the current state of the art employs circuits, semiconductors, and other electrical engineering tools, the team has come up with a game plan based on chemists’ ability to design custom atomic building blocks that control the properties of larger molecular structures when are placed together.

The findings, published last week in nature chemistryit could ultimately lead to a leap in quantum processing power.

“The idea is, instead of building a quantum computer, let chemistry build it for us,” said Eric Hudson, the David S. Saxon Presidential Professor of Physics at UCLA and corresponding author of the study. “All of us are still learning the rules for this kind of quantum technology, so this work is very science fiction at the moment.”

The basic units of information in traditional computing are bits, 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 a computer’s processing power. More than 1,000 normal bits are required to represent just 10 qubits, while 20 qubits require more than 1 million bits.

That feature, at the heart of quantum computing’s transformative potential, depends on counterintuitive rules that apply when atoms interact. For example, when two particles interact, they can become bonded or entangled, so that measuring the properties of one determines the properties of the other. Entanglement of qubits is a requirement of quantum computing.

However, this entanglement is fragile. When qubits encounter subtle variations in their surroundings, they lose their “quantity”, which is necessary 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 pieces of machinery.

To apply quantum computing in practice, engineers need to increase that processing power. Hudson and his colleagues believe they have taken a first step with the study, where theory guided the team to personalize molecules that protect quantum behavior.

Scientists developed small molecules including calcium and oxygen atoms and act like qubits. These calcium and oxygen structures form what chemists call a functional group, which means that it can connect to almost any other molecule and at the same time impart its own properties to that molecule.

The team showed that their functional groups they kept their desired structure even when attached to much larger molecules. Its qubits can also withstand laser cooling, a key requirement for quantum computing.

“If we can attach a quantum functional group to a surface or some long molecule, we could control more qubits,” said Hudson. “It should also be cheaper to scale up, because an atom is one of the cheapest things in the universe. You can make as many as you want.”

In addition to its potential for next-generation computing, quantum technology functional group could be a boon to basic discovery in chemistry and life sciencesfor example, by helping scientists discover more about the structure and function of various molecules and chemicals in the human body.

“Qubits can also be exquisitely sensitive tools for measurement,” said study co-author Justin Caram, an assistant professor of chemistry and biochemistry at UCLA. “If we could protect them so they can survive in complex environments like biological systems, we would be armed with a lot of new information about our world.”

Hudson said the development of a chemical-based quantum computer could take decades and is not certain to be successful. Future steps include anchoring qubits to larger molecules, persuasion qubits interact as processors without unwanted signaling, and weave them together to work as a system.

The project was started by a Department of Energy grant that provided physicists and chemists with the opportunity to cut through discipline-specific jargon and speak in a common scientific language. Caram also credits UCLA’s easy collaborative atmosphere.

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