Old-school materials could power quantum computing and reduce data center energy use

A new twist on classical matter could advance quantum computing and make modern data centers more energy efficient, according to a team led by Penn State researchers.

Barium titanate, first discovered in 1941, is known for its powerful electro-optic properties in bulk, or three-dimensional, crystals. Electro-optic materials such as barium titanate act as bridges between electricity and light, converting signals carried by electrons into signals carried by photons, or light particles.

However, despite its promise, barium titanate never became the industry standard for electro-optic devices, such as modulators, switches, and sensors. Instead, lithium niobate—which is more stable and easier to fabricate, even if its properties don’t quite measure up to those of barium titanate—fills that role instead. But that can change, according to Penn State professor Venkat Gopalan, Penn State professor of the study published in Advanced Materials, by replacing barium titanate with ultrathin strained thin films.

“Barium titanate is known in the materials science community as a champion material for electro-optics, at least on paper,” Gopalan said. “It has one of the largest known electro-optic property values ​​in its bulk, single-crystal form at room temperature. But when it comes to commercialization, it’s never made the leap. What we’ve done shows that when you take this classic material and just stress it in the right way, it can’t work.”

Critically, Gopalan said, the newly developed material improves the conversion of signal-carrying electrons to signal-carrying photons that has been shown to occur at cryogenic temperatures. Cryogenic operation is essential for quantum technologies based on superconducting circuits. However, transmitting information between distant quantum computers requires converting that information into light, where conventional fiber optics at room temperature can be used to enable true quantum networks.

Efficient power-to-optical transducers can also find use in data centers that support everything from artificial intelligence (AI) to online services. These facilities use a large amount of energy, much of it for cooling, a problem that optical links can help mitigate. These facilities use a large amount of energy, mostly for cooling. Because photons are particles of light, they can carry information without generating the kind of heat that moves electrons through wires, making them far more energy efficient.

“Overall integrated photonic technologies are becoming increasingly attractive to use large data centers to process and communicate large volumes of data, especially with accelerating AI tools,” said Penn State study co-author Aidan Ross, a graduate research assistant.

“The basic idea is that we can send information to these centers using photons instead of electrons, letting us send many streams of information in parallel, and without having to worry about heating up our electronics, the sheer infrastructure needed to keep such centers cool.”

The team assembled barium titanate into films about 40-40 nanometers thick, thousands of times thinner than a human hair. By growing the film on another crystal, the researchers forced the atoms into new positions, creating what scientists call a metastable phase, which is a crystal structure that does not naturally occur in the bulk form.

“Metastable phases may have properties that the stable version may not have,” Gopalan said. “In this case, the stable phase of barium titanate loses most of its electro-optic efficiency at low temperatures, which is a huge problem for quantum applications that require superconducting qubits.

Albert Soswa, co-leader of the study and a doctoral candidate in materials science and engineering, compared the concept to a ball resting on a hill.

“What we call a metastable phase refers to a crystal structure that is not the lowest-energy arrangement of atoms that the material wants to carry,” Sosova said.

“Everything in nature wants to exist in its lowest energy state. Think of a ball on a hill, it will naturally roll to the bottom of the hill. But if you cradle the ball in your arms, you’ve given it a new place until someone slips out of your hands, knocking the thing around nicely because it’s like holding the ball. At least until it’s disturbed.”

Along with more energy-efficient data centers, these findings could also address one of the biggest challenges in quantum computing: transferring information between quantum computers. Right now, researchers use microwave signals that decay quickly, making it difficult to send data over long distances.

“Microwave signals work for qubits on a chip, but they’re terrible for long-distance transmission,” Sosova said. “To go from individual quantum computers to quantum networks spread across multiple computers, information needs to be converted into a type of light that we are already very good at sending over long distances, such as the infrared light used for fiber-optic internet.”

The approach to strained barium titanate thin films can be applied to a wide range of materials, said Sankalpa Hazra, co-leader of the study and a doctoral candidate in the Department of Science and Engineering.

Next, the team is looking to extend their work beyond barium titanate.

“Achieving this result with barium titanate was a matter of taking a new material design approach to a very classic and well-studied material system,” Gopalan said. “Now that we better understand this design strategy, we have some understudied material systems to which we want to apply this same approach. We are very optimistic that some of these systems will even exceed the incredible performance that has come from barium titanate.”