The experiment takes place in a vacuum chamber, where ultracold atomic clouds with a temperature of 30 nK are generated via laser cooling. Pictured: Professor Herwig Ott and Dr. Erik Bernhart. Credit: RPTU, Thomas Koziel

The microscopic processes taking place in superconductors are difficult to observe directly. Researchers at the RPTU University of Kaiserslautern-Landau have therefore implemented a quantum simulation of the Josephson effect: They separated two Bose-Einstein condensates (BECs) by means of an extremely thin optical barrier.

The characteristic Shapiro steps were observed in the atomic system. The research was published in the journal Science.

Two superconductors separated by a wafer-thin insulating layer—that’s how simple a Josephson junction looks. But despite its simple structure, it harbors a quantum mechanical effect that is now one of the most important tools of modern technology: Josephson contacts form the heart of many quantum computers and enable high-precision measurements—such as the measurement of very weak magnetic fields.

This is used, among other things, in magnetoencephalography (MEG), i.e., in medical diagnostics for examining magnetic fields in the brain.

It is important to note that the processes that take place in a Josephson junction occur at the level of individual quanta and are difficult to observe directly in the superconductor.

To make the microscopic processes experimentally accessible, researchers use a trick known as quantum simulation. In general terms, this involves transferring a complex quantum system to another, more easily observable system. This allows effects to be investigated that are virtually inaccessible in their conventional environment.

At RPTU, an experimental research team led by Herwig Ott has now applied this principle to the Josephson effect.

Instead of superconductors, they used an ultracold gas of atoms, a so-called Bose-Einstein condensate. The researchers separated two such condensates by means of a very thin optical barrier, which was generated with a focused laser beam and moved periodically.

In this way, the researchers simulated what happens in a superconducting Josephson junction under microwave irradiation. The microwave radiation serves to generate an additional alternating current through the Josephson contact.

Shapiro steps are a universal phenomenon

The result of the quantum simulation was impressive: the characteristic Shapiro steps appeared in the atomic system—quantized voltage plateaus that serve to calibrate the electrical voltage. They depend only on natural constants and the frequency of the modulation and form the basis on which the voltage standard for the "volt" is realized worldwide.

"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," states Ott.

The study was conducted in collaboration with the theory groups of Ludwig Mathey from the University of Hamburg and Luigi Amico from the Technology Innovation Institute in Abu Dhabi. The work provides a textbook example of quantum simulation.

Ott summarizes, "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."

Using atoms to understand the world of electrons

In the future, Ott and his team want to connect several such "building blocks" with each other "to build real circuits for atoms." Instead of electrons, atoms would then flow through the circuit—a field of work known as "atomtronics".

"Such circuits are particularly well suited for observing coherent effects, i.e., wave-like effects," says Erik Bernhart, who conducted the experiments as a doctoral student. In addition, the movement of atoms can be "seen" directly in atomic circuits, which is much more difficult with electrons in solids.

"We also want to replicate other fundamental components known from electronics for our atoms and understand them precisely at the microscopic level."

More information: Erik Bernhart et al, Observation of Shapiro steps in an ultracold atomic Josephson junction, Science (2025). DOI: 10.1126/science.ads9061

Citation: Atomic Josephson contacts: How Bose-Einstein condensates replicate Shapiro steps (2025, December 12) retrieved 12 December 2025 from https://phys.org/news/2025-12-atomic-josephson-contacts-bose-einstein.html

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