Physicists made atoms behave like a quantum circuit

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.

In conventional devices, microwave radiation induces an additional alternating current through the Josephson contact. In the atomic version of the experiment, the moving laser barrier played the same role, allowing the team to closely mimic the behavior of electronic junctions using atoms instead.

Shapiro Steps Are a Universal Phenomenon

The results of the experiment were striking. The atomic system displayed clear Shapiro steps, which are quantized voltage plateaus used worldwide to calibrate electrical voltage. These steps depend only on fundamental constants and the frequency of the applied modulation, making them the foundation of the global voltage standard for the "volt."

"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 Herwig Ott.

Bridging the Quantum Worlds of Atoms and Electrons

The study was carried out in collaboration with theoretical physicists Ludwig Mathey from the University of Hamburg and Luigi Amico from the Technology Innovation Institute in Abu Dhabi. Together, the teams demonstrated how a well-known effect from solid-state physics can be faithfully reproduced in an entirely different setting.

The work serves as a textbook example of quantum simulation. As Herwig Ott explains, "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 Atomic Circuits to Explore Quantum Physics

Looking ahead, Ott and his colleagues plan to link multiple atomic junctions together to form complete circuits made of atoms. In these systems, atoms would move through the circuit instead of electrons, an emerging area of research known as "atomtronics."

"Such circuits are particularly well suited for observing coherent effects, i.e., wave-like effects," says Erik Bernhart, who carried out the experiments as part of his doctoral research. Unlike electrons in solid materials, atoms in these circuits can be directly observed as they move, providing a clearer view of quantum behavior. "We also want to replicate other fundamental components known from electronics for our atoms and understand them precisely at the microscopic level."