Researchers at RPTU University of Kaiserslautern-Landau have simulated a Josephson junction using ultracold atoms, revealing key quantum effects previously hidden in superconductors. By separating Bose-Einstein condensates with a moving laser barrier, they observed Shapiro steps, confirming the phenomenon's universality. The findings, published in Science, bridge atomic and electronic quantum systems.
Josephson junctions are vital in quantum technologies, enabling precise voltage measurements and forming cores of quantum computers. They consist of two superconductors separated by a thin insulator, but their quantum processes are hard to observe directly due to their microscopic scale.
To address this, a team led by Herwig Ott at RPTU University of Kaiserslautern-Landau employed quantum simulation with ultracold atoms. They created two Bose-Einstein condensates and divided them using a narrow optical barrier from a focused laser beam. By periodically moving this barrier, they mimicked the effect of microwave radiation on a traditional Josephson junction.
The experiment produced clear Shapiro steps—quantized voltage plateaus at multiples of the driving frequency. These steps, which underpin the global voltage standard, appeared in the atomic system just as in superconducting devices. "In our experiment, we were able to visualize the resulting excitations for the first time. The fact that this effect now appears in a completely different physical system—an ensemble of ultracold atoms—confirms that Shapiro steps are a universal phenomenon," Ott stated.
The study, conducted with theorists Ludwig Mathey from the University of Hamburg and Luigi Amico from the Technology Innovation Institute in Abu Dhabi, demonstrates how quantum simulation uncovers hidden physics. As Ott explained, "A quantum mechanical effect from solid-state physics is transferred to a completely different system—and yet its essence remains the same. This builds bridges between the quantum worlds of electrons and atoms."
Erik Bernhart, who performed the experiments during his doctoral research, highlighted future potential: "Such circuits are particularly well suited for observing coherent effects, i.e., wave-like effects." The team aims to connect multiple atomic junctions into circuits for atomtronics, allowing direct observation of atomic quantum behavior, unlike elusive electron movements in solids.
Published in Science (2025; 390 (6778): 1130), the work advances understanding of quantum universality and applications in fields like magnetoencephalography for brain imaging.