Using a nearly 20-feet tall microscope in a basement laboratory in Nieuwland Science Hall, Assistant Professor Xiaolong Liu and collaborators visualized quantum interference between two superconducting condensates, or states of matter where most particles share the same quantum state, inside a single material.

This first-time discovery, at the scale of individual atoms at extremely low temperatures, was published in Nature Materials. It advances understanding of unconventional multi-band superconductors and could help pave the way for next-generation quantum technologies. Liu used scanned Josephson tunneling microscopy to disentangle the two superfluid components, both of which act like the perfect conductor and carry electricity with zero resistance.

“Our new technique using scanned Josephson tunneling microscopy was the key,” said Liu, a researcher in the University of Notre Dame’s Department of Physics and Astronomy who is affiliated with the Stravropoulos Center for Complex Quantum Matter. “This is a capability that has never been possible.”

Understanding how these coexisting superfluids interact is crucial to the future of quantum technologies. Unlike classical computers, quantum devices rely on quantum states to perform calculations. However, many materials for quantum computing are poorly understood.

Josephson tunneling is a quantum effect where paired electrons, called Cooper pairs, pass between two superconducting materials through quantum mechanical tunneling. In most superconductors, there’s only one charged superfluid made of these electron pairs. But in this experiment, Liu and researchers worked with iron selenide, a material in which two superfluids coexist and interact.

He described Josephson tunneling to the two coexisting superfluids like listening to two people speaking at once. It's not easy to distinguish between them, and the result is a messy mixture of voices, he said. The ability to detect and disentangle tunneling contributions from each superfluid is the key result of the research. Within a sign-changing superconductor like iron selenide, the Josephson tunneling current tends to cancel each other out.

“Because they have different signs, the current flows actually in opposite directions so there is a quantum interference effect,” Liu said. “This interference effect was long predicted.”

Their result came thanks to the sophisticated, specially designed microscope housed in a soundproof, vibration-isolated room below Nieuwland. The room was isolated from the foundation of the building to protect it from noise that could ruin highly precise measurements, Liu said.

Using the microscope operating at millikelvin temperatures, researchers saw the two quantum fluids interacting with one another, creating patterns across the atomic surface of the material. In areas where one condensate was strong, the other was weak, and vice versa.

“They are kind of competing with each other,” Liu said. The atomic-level detail allowed the researchers to achieve what they call “condensate-resolved imaging,” which gave the researchers the ability to look at each superfluid separately, even though they occupy the same space.

The team hopes to adapt their technique to visualize even more complex superconductors with spatial modulations, according to Liu. “That’s something we are still trying to work on,” he said.

The research was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Science, and the National Science Foundation, Division of Materials Research.

Originally published by Deanna Csomo Ferrell at science.nd.edu on August 13, 2025.