Pioneering experiment could help design energy-efficient materials.

In a groundbreaking study published in Physical Review Research, a group of University of Chicago scientists announced they were able to turn IBM’s largest quantum computer into a quantum material itself.

They programmed the computer such that it turned into a type of quantum material called an exciton condensate, which has only recently been shown to exist. Such condensates have been identified for their potential in future technology, because they can conduct energy with almost zero loss.

“The reason this is so exciting is that it shows you can use quantum computers as programmable experiments themselves,” said paper co-author David Mazziotti, a professor in the Department of Chemistry, the James Franck Institute and the Chicago Quantum Exchange, and an expert in molecular electronic structure. “This could serve as a workshop for building potentially useful quantum materials.”

For several years, Mazziotti has been watching as scientists around the world explore a type of state in physics called an exciton condensate. Physicists are very interested in these kinds of novel physics states, in part because past discoveries have shaped the development of important technology; for example, one such state called a superconductor forms the basis of MRI machines.

Though exciton condensates had been predicted half a century ago, until recently, no one had been able to actually make one work in the lab without having to use extremely strong magnetic fields. But they intrigue scientists because they can transport energy without any loss at all—something which no other material we know of can do. If physicists understood them better, it’s possible they could eventually form the basis of incredibly energy-efficient materials.

“This could serve as a workshop for building potentially useful quantum materials.”

— Prof. David Mazziotti

To make an exciton condensate, scientists take a material made up of a lattice of particles, cool it down to below -270 degrees Fahrenheit, and coax it to form particle pairs called excitons. They then make the pairs become entangled—a quantum phenomenon where the fates of particles are tied together. But this is all so tricky that scientists have only been able to create exciton condensates a handful of times.

“An exciton condensate is one of the most quantum-mechanical states you can possibly prepare,” Mazziotti said. That means it’s very, very far from the classical everyday properties of physics that scientists are used to dealing with.

Enter the quantum computer. IBM makes its quantum computers available for people around the world to test their algorithms; the company agreed to “loan” its largest, called Rochester, to UChicago for an experiment.

Graduate students LeeAnn Sager and Scott Smart wrote a set of algorithms that treated each of Rochester’s quantum bits as an exciton. A quantum computer works by entangling its bits, so once the computer was active, the entire thing became an exciton condensate.

“It was a really cool result, in part because we found that due to the noise of current quantum computers, the condensate does not appear as a single large condensate, but a collection of smaller condensates,” Sager said. “I don’t think any of us would have predicted that.”

Mazziotti said the study shows that quantum computers could be a useful platform to study exciton condensates themselves.

“Having the ability to program a quantum computer to act like an exciton condensate may be very helpful for inspiring or realizing the potential of exciton condensates, like energy-efficient materials,” he said.

Beyond that, just being able to program such a complex quantum mechanical state on a computer marks an important scientific advance.

Because quantum computers are so new, researchers are still learning the extent of what we can do with them. But one thing we’ve known for a long time is that there are certain natural phenomena that are virtually impossible to model on a classical computer.

“On a classical computer, you have to program in this element of randomness that’s so important in quantum mechanics; but a quantum computer has that randomness baked in inherently,” Sager said. “A lot of systems work on paper, but have never been shown to work in practice. So to be able to show we can really do this—we can successfully program highly correlated states on a quantum computer—is unique and exciting.”

Reference: “Preparation of an exciton condensate of photons on a 53-qubit quantum computer” by LeeAnn M. Sager, Scott E. Smart and David A. Mazziotti, 9 November 2020, Physical Review Research.DOI: 10.1103/PhysRevResearch.2.043205

Funding: U.S. Department of Energy Office of Basic Energy Sciences, National Science Foundation, U.S. Army Research Office.