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

(Press-News.org) UNIVERSITY PARK, Pa. — A new twist on a classic material could advance quantum computing and make modern data centers more energy efficient, according to a team led by researchers at Penn State.   

Barium titanate, first discovered in 1941, is known for its powerful electro-optic properties in bulk, or three-dimensional, crystals. Electro-optic 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 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 with those of barium titanate — filled that role instead. But by reshaping barium titanate into ultrathin strained thin films, this could change, according to Venkat Gopalan, Penn State professor of materials science and engineering and co-author of the study published in Advanced Materials.   

“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 electro-optic property values known in its bulk, single crystal form at room temperature. But when it comes to commercialization, it never made the leap. What we have done is show that when you take this classic material and strain it in just the right way, it can do things no one thought possible.” 

Critically, Gopalan said, the newly formed material improves the conversion of signal-carrying electrons into signal-carrying photons by over ten times what has been shown at cryogenic temperatures. Cryogenic operation is necessary for quantum technologies based on superconducting circuit. However, the delivery of information between distant quantum computers requires the conversion of that information into light, where traditional fiber-optics at room temperature could be used to enable true quantum networks. Efficient electrical-to-optical transducers can also find use in data centers that support everything from artificial intelligence (AI) to online services. These facilities consume vast amounts of energy, much of it to stay cool, a problem that optical links can help mitigate These facilities consume vast amounts of energy, much of it to stay cool. Because photons are particles of light, they can carry information without generating the kind of heat that moving electrons through wires does, making them far more energy efficient. 

“Integrated photonic technologies as a whole are becoming increasingly attractive to companies that use large data centers to process and communicate large data volumes, especially with the accelerating 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 throughout these centers using photons rather than electrons, letting us send many streams of information in parallel, and do so without having to worry about our electronics heating up, the sheer infrastructure needed to keep such centers cool and so on.” 

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 occur naturally in bulk form.  

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

Albert Suceava, co-lead author of the study and 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 that material wants to take on,” 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 foot of the hill. But if you cradle the ball in your arms, you’ve given it a new place it can rest until someone comes along and gives you a push, 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’ve done something to the material that makes it okay with taking on this new structure, at least until it’s disturbed.”  

Along with more energy efficient data centers, the findings could also address one of the biggest challenges in quantum computing: moving information between quantum computers. Right now, researchers use microwave signals that fade quickly, making it hard 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 go from individual quantum computers to quantum networks spread over multiple computers, information needs to be converted into a kind of light that we’re already very good at sending long distances, such as the infrared light used for fiber optic internet.”  

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

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

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

Along with Gopalan, Ross, Hazra and Suceava, other authors of the study include graduate students Ian Reed Philippi, graduate student in materials science and engineering; Brynn Brower, doctoral candidate in physics; Lei Ding, graduate student in materials science and engineering; Yingxin Zhu, graduate student in engineering science and mechanics; Zhiyu Zhang, graduate student in engineering science and mechanics; Himirkanti Sarkar, doctoral candidate in materials science and engineering; and Saugata Sarker, graduate research assistant in materials science and engineers, Yang Yang, assistant professor of engineering science and mechanics; Vladimir A. Stoica, research associate professor of materials science and engineering; and Long-Qing Chen, Donald W. Hamer Professor of Materials Science and Engineering, all from Penn State. Other co-authors include Dylan Sotir, Suchismita Sarker and Darrell G. Schlom, all from Cornell University.  

The U.S. National Science Foundation and the U.S. Department of Energy supported this research. 

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