Old-school material could power quantum computing and cut data center energy use

A new version of a classic material 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-optical properties in bulk or three-dimensional crystals. Electro-optical materials like barium titanate act as bridges between electricity and light, converting signals carried by electrons into signals carried by photons or particles of light.

However, despite its promise, barium titanate never became the industrial standard for electro-optical devices, such as modulators, switches, and sensors. Instead, lithium niobate, which is more stable and easier to manufacture, although its properties are not quite on par with those of barium titanate, instead fulfilled this role. But by reshaping barium titanate into ultra-thin thin films, that could change, according to Venkat Gopalan, professor of materials science and engineering at Penn State and co-author of the study published in Advanced Materials.

“Barium titanate is known in the materials science community as a champion electro-optical material, at least on paper,” Gopalan said. “It has one of the highest electro-optical property values ​​known in its single-crystal form at room temperature. But when it came to commercialization, it never made the big leap. What we did was show that when you take this classic material and filter it in the right way, it can do things that no one thought possible.”

According to Gopalan, the newly formed material improves the conversion of signal-carrying electrons into signal-carrying photons by more than ten times what has been demonstrated at cryogenic temperatures. Cryogenic operation is necessary for quantum technologies based on superconducting circuits. However, transmitting information between remote quantum computers requires converting that information into light, where traditional room-temperature optical fibers could be used to enable true quantum networks.

Efficient electrical-optical transducers can also be used in data centers that support everything from artificial intelligence (AI) to online services. These facilities consume large amounts of energy, much of it to stay cool, a problem that optical links can help alleviate. These facilities consume large amounts of energy, much of it to keep cool. Because photons are particles of light, they can carry information without generating the kind of heat that moving electrons through wires produces, making them much more energy efficient.

“Integrated photonics technologies as a whole are becoming increasingly attractive to companies that use large data centers to process and communicate large volumes of data, especially with the accelerated adoption of AI tools,” said Aiden Ross, co-lead author of the study and graduate research assistant at Penn State.

“The basic idea is that we could send information to these centers using photons rather than electrons, which would allow us to send many streams of information in parallel, and do so without having to worry about our electronics heating up, the infrastructure needed to keep these centers cool, etc.”

The team manipulated barium titanate into films about 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 exist in bulk form.

“Metastable phases can have properties that the stable version cannot possess,” Gopalan said. “In this case, the stable phase of barium titanate loses much of its electro-optical performance at low temperatures, which is a big problem for quantum applications requiring superconducting qubits. But the metastable phase we created not only avoided this drop, but also showed an exceptional response.”

Albert Suceava, co-lead author of the study and a doctoral student 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 this material wants to take,” Suceava 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 hold the ball in your arms, you have given it a new place where it can rest until someone comes and pushes you, knocking that ball out of your hands so it can roll down the hill. The metastable phase is like holding the ball, it only exists because we did something at material that allows it to accept this new structure, at least until it is disturbed.”

In addition to more energy-efficient data centers, the results could also address one of the biggest challenges in quantum computing: moving information between quantum computers. Currently, researchers use microwave signals that fade quickly, making it difficult to send data over long distances.

“Microwave signals work for qubits on a chip, but they are terrible for long-distance transmission,” Suceava said. “To move from individual quantum computers to quantum networks spread across multiple computers, information must be converted into a kind of light that we are already very good at sending over long distances, like the infrared light used for fiber-optic Internet.”

Sankalpa Hazra, co-lead author of the study and a doctoral student in materials science and engineering, said the strained barium titanate thin film approach could be applicable to a wide range of materials.

The team is next looking to expand its work beyond barium titanate.

“To achieve this result with barium titanate required a new approach to materials design versus a very conventional, well-studied materials system,” Gopalan said. “Now that we better understand this design strategy, we want to apply the same approach to some less well-studied material systems. We are very optimistic that some of these systems will even surpass the incredible performance of barium titanate.”