Quantum staircase effect seen in ultracold atomic junction

by Robert Schreiber
Berlin, Germany (SPX) Dec 26, 2025

Scientists have observed Shapiro steps, a staircase-like quantum effect, in a system of ultracold atoms driven by an alternating current across an atomic Josephson junction formed by atoms cooled near absolute zero and separated by a thin laser-light barrier.

In the experiment, the atoms collectively crossed the laser barrier without energy loss, as if it were transparent, through quantum tunneling, while the chemical potential difference between the two atomic reservoirs increased in discrete, evenly spaced steps rather than changing continuously.

The step height was set directly by the frequency of the applied alternating current, establishing these quantized chemical potential jumps as the atomic analogue of Shapiro steps known from conventional solid-state Josephson junctions.

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

"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 the Technology Innovation Institute.

"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."

The findings show that ultracold atomic systems provide a versatile platform for exploring fundamental quantum phenomena and for studying and exploiting the collective dynamics of quantum systems in a highly controlled setting.

Research Report:Shapiro steps in strongly-interacting Fermi gases