Josephson junctions play a central role in modern physics and technology. They enable extremely precise measurements, define the international standard for electrical voltage, and serve as essential components inside many quantum computers. Despite their importance, the quantum-scale processes occurring inside superconductors are notoriously difficult to observe directly.

To overcome this challenge, researchers at the RPTU University of Kaiserslautern-Landau turned to quantum simulation. Instead of studying electrons inside a solid material, they recreated the Josephson effect using ultracold atoms. Their approach involved separating two Bose-Einstein condensates (BECs) with an exceptionally thin optical barrier created by a focused laser beam that was moved in a controlled, periodic way. Even in this atomic system, the defining signatures of Josephson junctions emerged. The experiment revealed Shapiro steps, which are distinct voltage plateaus that appear at multiples of a driving frequency, just as they do in superconducting devices. Published in the journal Science, the work stands as a clear example of how quantum simulation can uncover hidden physics.

Why Josephson Junctions Matter

At first glance, a Josephson junction has a simple structure. It consists of two superconductors separated by an extremely thin insulating layer. Yet this basic setup produces a powerful quantum mechanical effect that underpins some of today's most advanced technologies. Josephson contacts form the core of many quantum computers and make it possible to measure extraordinarily weak magnetic fields.

These measurements are crucial in applications such as magnetoencephalography (MEG), a medical imaging technique used to detect magnetic signals generated by activity in the human brain. The precision of Josephson junctions is what makes such sensitive diagnostics possible.

Making Invisible Quantum Effects Observable

The challenge with Josephson junctions is that their behavior unfolds at the level of individual quanta. Inside a superconductor, these microscopic processes cannot be easily tracked or visualized. To study them in detail, physicists rely on quantum simulation, a strategy that maps a complex quantum system onto a different one that is easier to control and observe.

By recreating the essential physics in a new environment, researchers can explore effects that would otherwise remain hidden. This approach allows scientists to test fundamental ideas and confirm whether certain behaviors are truly universal across different physical systems.

Recreating the Josephson Effect with Ultracold Atoms

At RPTU, an experimental team led by Herwig Ott applied quantum simulation directly to the Josephson effect. Rather than using superconductors, they worked with an ultracold gas of atoms known as a Bose-Einstein condensate. Two such condensates were separated by a narrow optical barrier formed by a focused laser beam. By moving this barrier periodically, the researchers recreated conditions similar to those in a superconducting Josephson junction exposed to microwave radiation.