December 23, 2025 -- For the first time, scientists have observed the iconic Shapiro steps, a staircase-like quantum effect, in ultracold atoms. In this experiment, an alternating current was applied to a Josephson junction formed by atoms cooled to near absolute zero and separated by an extremely thin barrier of laser light. Remarkably, the atoms were able to cross this barrier collectively and without energy loss, behaving as if the barrier were transparent, thanks to quantum tunneling. As the oscillating current flowed through the junction, the difference in chemical potential between the two sides did not change smoothly but instead increased in discrete, evenly spaced steps, like climbing a quantum staircase. The height of each step is directly determined by the frequency of the applied current, and these step-like chemical potential differences are the atomic analogue of Shapiro steps in conventional Josephson junctions.

The experimental team at the European Laboratory for Non-Linear Spectroscopy (LENS) in Sesto Fiorentino, Italy, carried out the study in collaboration with researchers from the National Institute of Optics (CNR-INO), the University of Florence, the University of Catania, the Technology Innovation Institute (TII) in Abu Dhabi, and the National Autonomous University of Mexico (UNAM). A complementary study carried out at RPTU University of Kaiserslautern-Landau was also published in the same issue of Science in a back-to-back format.

"Josephson junctions in solid-state superconducting platforms are already fundamental building blocks of quantum sensors and quantum computers, and were highlighted by the 2025 Nobel Prize in Physics as key tools for exploring quantum phenomena on macroscopic scales", says Giacomo Roati, Director of Research at CNR-INO and leader of the LENS experimental team. "In their ultracold-atom realization, these junctions offer unprecedented control, allowing us to directly probe the microscopic mechanisms that give rise to their macroscopic behaviour."

"Thanks to the high degree of control and precision in manipulating the atoms, we were able to uncover the physical synchronization mechanism responsible for the emergence of Shapiro steps in atomic Josephson junctions," explains Giulia Del Pace, researcher at the University of Florence and first author of the study. "This represents a crucial step in understanding how microscopic quantum behaviour gives rise to macroscopic phenomena."

"This is a major step for atomtronics," adds Luigi Amico, leader of the theoretical group that predicted the effect at the University of Catania and TII. "Like electrical currents in conventional electronics, atomtronic circuits guide neutral atoms with lasers, offering precise control for new quantum devices and applications in simulation, sensing, and technology."

These results demonstrate that ultracold atoms are not only an ideal platform for exploring fundamental quantum phenomena but also a powerful tool for investigating and harnessing the collective dynamics of quantum systems in a highly controlled environment.